If they aren't present anywhere else, you can shut off all your pumps while you remove the rock, and dip it in flatworm exit. You can also treat the entire tank with flatworm exit, but crustaceans will suffer. Try searching "flatworm exit". If the RC search function is tied up, I think you can google it also, and you'll be led to some of the threads. Lots of info out there. FE is a Salifert product.
What I've found to work almost as well is to just step up water circulation for short periods. They don't do well in higher water movement scenarios, so a periodic blast with a turkey baster seems to keep them in check. By periodic, I mean daily for a week or so. Then just whenever you see that they are coming back. It doesn't erdicate them, but seems to keep them at un-plague-like levels. The turkey baster treatment is also good for keeping your rock clear of debris and really should be done anyway in low-flow scenarios.