Corynactis. They're actually closer to corallimorphs (mushrooms) than anemones. The ones we get as hitch hikers generally stay fairly small, and although they may spread, they usually keep to the darker nooks and crannies in our tanks (places we don't generally keep "desirable" corals).
I've got a bunch in one tank (mostly a very light pinkish) and a few in another (yellow) and never had a problems with them in either tank.
FWIW,
Kevin