That could just be mold already in your environment feeding on what it needs, which could be salts or sugars.
I'm diabetic, so my urine has an above average amount of sugars and some other chemicals. I have to clean my toilets more frequently because of mold growth, and it starts off as black spots. EDIT: This is of course vital information you all needed to know.
I doubt any tap water is more corrosive than seawater. Seawater is brutal.
To be fair I've no idea if it was updated using ChoidujourNX, but that look like it. I also can't seem to use Daybreak on a SX Core so lol. Also pardon if talking about the switch here makes you uncomfortable