It's an Asterina star. The vast majority are harmless, film algae and bacteria eaters - that looks like one of the harmless ones. There is a specie of Asterina that are coral munchers - I've only ever seen those a time or two (rare, versus the harmless kind).
They reproduce by fission - they split in two, so it's not unusual to see them with some normal-length arms, and some short ones - after they split, they regenerate.
They can become quite numerous. Some people aren't bothered by that, some people are - if you're inclined to be bothered by it - remove it now and any others you find, before it multiplies.
In instances where they are really numerous, people use Harlequin Shrimp to get rid of them - the shrimp eats the tube feet off most sea stars, and a pair of Harlequins can wipe out a population of Asterinas. Once you run out of food for the Harlequins, you'll either have to buy sea stars for it to eat, or rehome it.
Jenn
>
