Been on my mind. Just curious what this was and what causes it. I'm assuming it is a defensive reaction, but don't know.

I removed a ton of rock from my tank last week. I get yellow sponge looking stuff growing all over my rocks and proceeded to scrub it off. The water turned blue like ink from broken a blue pen in the water.

Anyone know? This is rubbery feeling stuff, so may not be sponge. Turns black when dry.