m0useanswered grow question 8 months ago could be dried salts/minerals from the fertilizer or hard water.
Molds tent to be more fuzzy but also they are present in the grow and will be everywhere. watering less frequently and when its needed is how you control the spread.
If it bothers you spray a bit of peroxide on it, but not a lot or that will soak in and get the roots. peroxide will degrade naturally and won't leave any harmfull leftovers like bleach.