Green palys. I have some of the other "pests" in my tanks. Xenia, GSP, Anthelia you name it. They all have their own rock and don't spread to anything else. However, a long time ago I added a small colony of green palythoas to my tank, now it's popping up in completely random locations in the tank. It isn't a disaster yet, but there will come a day that they take over everything. I need to plan measures to eradicate them eventually, or at least contain them.