That looks like a ball anemone. If I remember correctly they are not actually anemones but a coralimorph. Purists will say they are pests but in my opinion they are much less invasive and pose less of a threat than aiptasia and majano anemones. Some of them fluoresce under actinic leds, so that's not too bad. The ones I've had in my tank tend to grow in the shadow of other corals or in dark crevasses of the tank and never became an issue. If you don't like it, remove it. Otherwise leave it be. If my ID turns out to be wrong, please disregard for now but save the description for the future.
I looked up tunicate and I don't think that is it... Sorry the picture is crappy, took it on my phone. It seems to have a long soft body that is spiraling around itself and I noticed yesterday a web like substance that seems to be coming from its mouth. It doesn't have a shell and its face is dark with two antenna. Hope that gives a better idea. Its in the back corner of my tank that doesn't get much action, and its been on top the same shell since I noticed it.
