I have about a half dozen of them, ranging from the size of a quarter to the size of a tea cup saucer. All are very hardy, easy to care for, like quite a bit of light, and once situated have never moved in over a year in my tank. When they were on the move trying to find their "spot", what corals they did touch were just fine afterwards.
Can't say for sure whether they will eat a fish or an invert, but most are so small I doubt they even have the capacity. These are not large hadoni's(sp?) or BTA's, most are the size of a quarter and pretty much stay that size.