I know exactly what those things are. I had the same problem about a month ago. They almost took over my tank. If they are mostly concentrated on a couple of rocks I took the rocks out and hosed them off outside, then let the rocks sit outside for a few days. If they aren't concentrated on a few of the rocks the only other thing you can do is get some hemostats or even some needlenose pliers (hemostats work best though) and pluck them off the rock. It will take a while depending on how many you have but it works very well. Like I said, I had a ton of them a month ago, and now 99% of them are gone.