What are these whitish deposits on the top surfaces of these houseplant leaves and should I do something about them?

Question

A few months ago I had scale bugs on a different houseplant in a different room and asked about them in a different SE site:

• Are these little white fuzzy insects "scale"? What species and what is their lifecycle like?

Looking under a low power microscope I also found some flat waxy like white deposits on the undersides of leaves that seemed to show other stages in the lifecycle, something that appeared to be hibernating/transforming, and much smaller, very tiny little black crawlers only visible in the low power microscope.

Today I found these irregular but mostly circular white, waxy-looking spots on the TOP sides only of a few leaves of this houseplant.

I occasionally give this one a direct dose of sun (as shown in the last photo) but mostly keep it in a moister, indirectly lit area. I also often water it using a plastic squirt bottle from a distance of a meter or two, so it frequently gets splashed on the top leaves by tap water.

So I'm wondering if maybe my tap water and/or plastic squirt bottle have stuff in it that forms these white spots when the splashed water droplets dry, but still I'm panicking, so I'd like to ask:

Question: What are these whitish deposits on the top surfaces of these houseplant leaves and should I do something about them?

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Click any image for full size:

Plant #1

UPDATE Here are new photos

since three more plants have suddenly exhibited the same whitish spots overnight, and squirt bottle I've been using for the last few months

Plant #2

Plant #3

Plant #4

Answer 1

The water you are misting the plants with also contains dissolved minerals. The composition and amount depends on where the water is from. My area has a lot of limestone so the tap water contains dissolved compounds of calcium.

After the water evaporates the minerals are left. They dry to form the white residue you see.

You can resolve this by:

• not misting the leaves. This is not a significant source of humidity and you can provide the same effect by letting the pot sit on top of a dish of water with some pebbles
• or use distilled water which has much lower levels of minerals

The residue can be removed with a clean cloth and gently rubbing the leaf. Difficult spots can be removed by using the cloth with a slightly acid solution like milk or watered down vinegar followed by a rinse to remove all residues

Edit: the fuzzy white stuff you asked about in another question are mealy bugs that can be controlled with soap and water and persistent control (every 5 to 6 days for at least three times maybe more)