My husband is obsessed with his new tank! He's just starting out with a new 50 gal tank and lots of help from the fish store. Right now he just has the rock (which changes daily??), two little "flushers" (store term, not mine! 8O ) and about 10 little crabs. He's only a few weeks into it. I know nothing about it all, but am amazed at the complexity of it all. Right now it seems that every day the rock he has in there changes in some way -- things growing on it, colors appearing, etc. So far the fish and crabs seem pretty happy and things are going well (according to my husband). There is some kind of cottony looking white stuff forming on some of the rock. Is that good or bad? What is it?