The answer to your second question, "how prevent it from growing in the other 3", is as follows:

• Move this plant away from the other three
• Disinfect your tools every time you've used on the diseased plant
• If possible, disinfect your hands too.

After you have saved the other plants, we can examine your first question, "How to kill the mold in the pot". Starting as an immediate measure, I agree with the suggestions made by Kevinsky. Id est, removing manually the mold.

But we do not have data on what this mold is. It could be an algae (Cyanobacteria), or a mold (fungi, like Phytophthora citrophthora, Penicillium italicum, or other else) or some lichen (fungi + algae). The colour is the same, more or less. To distinguish among them, we can start a cursory examination of view (texture and presentation are different), but an exact result we can have, possibly, from a microscopic examination.

Different causes require different remedies. I believe that a thorough examination would probably be too complicated. But certainly, I think, the mold is not only on the surface but also inside the ground.

So, to help you if you feel like it, you can

• Remove the plant from the pot
• Completely change pot and ground (for algae and lichens)
• Disinfect the bare roots and the stem with a fungicide (for fungi)
• Plant again your apple tree in an uncontaminated environment.

Try to do this quickly, without subjecting the roots to temperature changes or drafts. In this way, you deal with all the issues from various sides and it is possible, I believe, to solve them. This is what I would do.

The answer to your second question, "how prevent it from growing in the other 3", is as follows:

• Move this plant away from the other three
• Disinfect your tools every time you've used on the diseased plant
• If possible, disinfect your hands too.

After you have saved the other plants, we can examine your first question, "How to kill the mold in the pot". Starting as an immediate measure, I agree with the suggestions made by Kevinsky. Id est, removing manually the mold.

But we do not have data on what this mold is. It could be an algae (Cyanobacteria), or a mold (fungi, like Phytophthora citrophthora, Penicillium italicum, or other else) or some lichen (fungi + algae). The colour is the same, more or less. To distinguish among them, we can start a cursory examination of view (texture and presentation are different), but an exact result we can have, possibly, from a microscopic examination.

Different causes require different remedies. I believe that a thorough examination would probably be too complicated. But certainly, I think, the mold is not only on the surface but also inside the ground.

So, to help you if you feel like it, you can

• Remove the plant from the pot
• Completely change pot and ground (for algae and lichens)
• Disinfect the bare roots and the stem with a fungicide (for fungi)
• Plant again your apple tree in an uncontaminated environment.

Try to do this quickly, without subjecting the roots to temperature changes or drafts. In this way, you deal with all the issues from various sides and it is possible, I believe, to solve them. This is what I would do.

The answer to your second question, "how prevent it from growing in the other 3", is as follows:

• Move this plant away from the other three
• Disinfect your tools every time you've used on the diseased plant
• If possible, disinfect your hands too.

After you have saved the other plants, we can ask how to save this plant. That is, we can try to answer the first question, "How can kill the mold in the pot". Starting as an immediate measure, I agree with the suggestion made by Kevinsky. Id est, removing manually the mold.

But we do not have data on what this mold is. It could be an algae (Cyanobacteria), or a mold (fungi, like Phytophthora citrophthora, Penicillium italicum, or other else) or some lichen (fungi + algae). The colour is the same, more or less. To distinguish among them, we can start a cursory examination of view (texture and presentation are different), but an exact result you can have, possibly, from a microscopic examination.

Different causes require different remedies. I believe that a thorough examination would probably be too complicated. But certainly, I think, the mold is not only on the surface but also inside the ground.

So, to help you if you feel like it, you can

• Remove the plant from the pot
• Completely change pot and ground (for algae and lichens)
• Disinfect the bare roots and the stem with a fungicide (for fungi)
• Plant again your apple tree in an uncontaminated environment.

Try to do this quickly, without subjecting the roots to temperature changes or drafts. In this way, you deal with all the issues from various sides and it is possible, I believe, to solve them. This is what I would do.

The answer to your second question, "how prevent it from growing in the other 3", is as follows:

• Move this plant away from the other three
• Disinfect your tools every time you've used on the diseased plant
• If possible, disinfect your hands too.

After you have saved the other plants, we can examine your first question, "How to kill the mold in the pot". Starting as an immediate measure, I agree with the suggestions made by Kevinsky. Id est, removing manually the mold.

But we do not have data on what this mold is. It could be an algae (Cyanobacteria), or a mold (fungi, like Phytophthora citrophthora, Penicillium italicum, or other else) or some lichen (fungi + algae). The colour is the same, more or less. To distinguish among them, we can start a cursory examination of view (texture and presentation are different), but an exact result we can have, possibly, from a microscopic examination.

Different causes require different remedies. I believe that a thorough examination would probably be too complicated. But certainly, I think, the mold is not only on the surface but also inside the ground.

So, to help you if you feel like it, you can

• Remove the plant from the pot
• Completely change pot and ground (for algae and lichens)
• Disinfect the bare roots and the stem with a fungicide (for fungi)
• Plant again your apple tree in an uncontaminated environment.

Try to do this quickly, without subjecting the roots to temperature changes or drafts. In this way, you deal with all the issues from various sides and it is possible, I believe, to solve them. This is what I would do.