That isn't mold, it's moss/algae. You can tell (by the green) that it's growing from energy obtained by photosynthesis, so it's not a big threat competition-wise. It's a sign that the mix is constantly moist on the surface, and also grows faster in high humidity.
It will not damage the cactus seedlings, but here's a tip. When cacti germinate, they grow very slowly/remain the same size for a long time. That's because they're developing a strong root system. A sign they are established is that they will begin to grow at a faster rate. This means the root system is growing deep, and you can allow the top 1/2" of mix to dry between waterings. This will keep moss from establishing.
For now, you can use a toothpick and stir up the top layer if you want, to slow the moss, as described here:
Or you can use a pair of tweezers and carefully remove the moss. In the long run, it really doesn't matter. The moss won't be a problem after the seedlings mature a little, and you don't have to water so frequently.
And by the way, those seedlings look great! I hope you have success with them.