If the anemone looks like it's dying (ie. not attached, torn foot, disintegrating), get it out of your tank immediately because it can totally pollute your tank & kill off a bunch of your livestock. Saw one devastate the display tank at a LFS, & it wasn't pretty. Just turns to mush.
Anthony
Rule of thumb. If an anemone is not staying put & wandering the tank, it's an unhappy critter & something is wrong (either with the anemone or the tank setup). A happy anemone is an attached anemone.