if you like them and want to keep them, or they are too difficult to remove and removing the rock is not an option, just put some frogspawn next to where you don't want it to go/spread. This may sound silly but has worked great for me. The frogspawn will keep it at bay for sure. in one of my old tanks I had some of those wood polyps that were spreading like crazy until I put the frogspawn in its way. it held them there for almost 2 years. I also had something else in one of my tanks (kinda like yellow palys/ mini anemones). They were actually quite pretty when they first showed up, but very quickly they started multiplying across the rock that they were on and its apparent that they weren't slowing down. Enter the frogspawn once again and they were dead in their tracks for a long time. The best is just to remove the whole rock, but like in my case the rock they were on was a major support rock with about 100 lbs LR stacked on top of it, in an established tank.