Hard to tell from the picture, but it looks like it could be corynactis or pseudocorynactis (which includes the ball anemones mentioned above). Not really an anemone, but a corallimorph (more of a mushroom). Hard to tell from your picture, but the hallmark of these little hitchhikers is a white ball-tip at the end of the tentacles. They are often translucent, with colors ranging from red to orange to pink to yellow, but can be opaque in a variety of colors as well.
The larger ones (which can be sold in the trade as orange ball anemones) tend to be singles, while the smaller variety can multiply, but generally stay in fairly cryptic areas of the tank (either out of direct light, or come out more after lights out). I've had quite a variety of them in my tanks and they have never caused any problems with other sessile invertebrates.
HTH,
Kevin