The anemone that came on your hermit crab is a simple anemone, and it is harmless. It will not propogate and will likely die on it's own if the hermit is no longer carrying it around. They only live on hermit crab shells, as this particular species has developed a sort of symbiotic relationship with the hermits. They provide some camoflage, and in return they are carried around with the hermit, even when he changes shells. This provides the anemone with some protection and a varying food source which is beneficial. It will not harm anything in your tank, and as long as it stays alive, will simply be an interesting conversation piece.