If you're seeing one, they're are likely many, many more that you're not seeing. Under the right conditions, they breed rapidly.
Personally, I like them. I actually go out of my way to introduce them to my tanks. They're excellent detrivores, and a valued part of my cleanup crew. If they get way out of hand, a Harlequin Shrimp will knock them back. The reef club I belonged to years ago had a pair of them we'd move around from tank to tank, as needed.
On the other hand, some people have ended up with species of these things that are documented coral eaters. From what I've seen, if the thing is white to off white on top, you're probably safe. If it is mottled brown on top, it could well be one of the coral predators. Can't really blame folks who have had this problem for wanting to eradicate them entirely.