roka64 said:Can you get a pic? Sounds like it could be an encrusting sponge, it is growing on the top or bottom? It's hard to guess with no pic.
Good point, was the rock colorful before, or are you using base rock?Ziggy953 said:Ok..I'm going to say it is what was coraline algae. When it is exposed to air for a period of time it dies and bleaches or turns white. Given time and the right lighting it will grow back.
MT79 said:Usually it will come back after cycling, w/ proper care and light. It tends to grow back/cover areas of rock that were once already covered, and other relatively smooth surfaces, fastest