I have them on my rocks in the sump.. Only an established tank will grow the different colors os sponge like that.. I have purple, blue, yellow, black.. All different color sponge.. Not a bad thing most people wish they had it..
Not so true.
I have an encrusting yellow sponge and it is invading. Moves onto my candy canes and have smother a few colonies already.
Large & dwarf angels do not seem to pick on this sponge.
I'd advise leaving them there unless you have a problem with them. They add to your tank's biological filtration capacity, which is always a good thing.