Well, its a good thing, kinda. Depends on what you have in the tank that can keep it under control when it gets to be a bit much. But, it help polish the water, give PODs a place to breed. Not a bad thing, but ya gotta watch the growth.
I am battling the same problem. I keep scrubbing my rocks and it just keeps coming back, I don't know what to do about it. I am like you, I have about 10 crabs and they are not making a dent in it, what else can we do. There must be something.