In all likelihood, it is the common Asterina star. Loads of info if you search under that name. They tend to have 5-7 arms, some of which are usually unequal. They reproduce through splitting. They are quite common in most tanks, and are generally considered to eat algal and bacterial films on the rocks and glass. however, some (especially GARF) have claimed that all of these guys are coral eaters, something that I, and most other invert zoo people in the hobby like Dr. Ron, and Dr. Toonen, as well as seastar specialists, would refute. Most do not cause any trouble at all.
There are probably a few species, and you should keep an eye on them, but they are impossible to id based on size, color, number of arms, etc. They will certainly be drawn to stressed, dead or dying corals as well, and I believe this is the source of some of their reputation. However, I have them reproducing nicely in my nano tanks without any sps corals at all. Some will also say that they eat coralline, which they may very well do, but typically this is not an issue in reef tanks that are doing well.
Keep an eye on them, but the overwhelming majority are fine additions to a tank.
