I don't think you will have to worry too much about the little brittlestars. Odds are there are far more in there than the one you saw...and you only need one anyway because they can handle reproduction on their own.
Most people never see them at all, and then they see a bunch of them in the sand. But you have to know what to look for. Most of the time, you will not see them.
As for the other stars, these are also hitchkikers. They are not sand sifters. They are
Asterina stars, and a search for this term should result in many hits (at least one a week!). There are a few species imported, but they are very difficult to distiguish. Most people have a species that is an algal/bacterial film grazer, and will randomly cover the rocks and glass.
A few people have the species that eats SPS corals. In this case, it will be on the corals, and not just randomly on the glass/rocks. So just keep an eye out if you have any. Otherwise, the majority are pretty good to have, IMO. They reproduce by fission, splitting in half, and tend to have uneven legs (and from 5-7 of them). Again, they will generally only be about the size of a nickel or quarter at most.