The common hitch hiker seastar is Asterina, and you commonly do not notice them for awhile. They reproduce easily by splitting, and do not get much larger than a nickel or so. A search should turn up loads of info, though sadly perhaps not pictures yet.
I would also search for sand sifter star, one I do not recommend as most are doomed to die of starvation in 6 or so months - perhaps up to a year - depending on tank size. I strongly recommend you see if it takes to spot feeding (anything meaty placed near an arm).