When I left home this morning the live rock looks fine but when I came back at night I found some brown stuff growing on the rock. Can someone tell me what is it? Is it harmful for the fishes?
I'd suck out as much as you can using a syphon hose. Try not to let it break up when you remove it, or it can spread around the tank. I'd also step up water changes
i tried to take out the live rock in hope to manually remove them, but as soon as I took it out of the water, everything retracted and nothing can be removed...