well, petri dishes work by culture (bacteria are living, they need food), and theres plenty of chemicals the react with water. drain and refill, put a very bright light and see if something is deposting the substance, (side note: it sounds kinda like hagfish slime, probably not though)I didnt get a good picture. Was in too big of a rush to drain and clean. Unsure what type of chemical reaction would cause it in rodi water…
I can probably get better pics the next time i drain it. Parts looked like frozen condensation ice shards that grow on your glass, parts were circular. Google “bacteria petri”. That’s what it looked like.