Agree with the kalk paste, but that one looks easy enough to grab at the base with a long pair of tweezers, twist, and pull off your rock. I've found manual removal works pretty well on some colonies, but just make sure you don't let any stragglers float around. I have several colonies in my tank, and one particular one that settled right next to my torch coral. After a couple months next to the torch, it was gone. Hydroids 0, Torch 1. While they aren't the nicest things to have, there's plenty of things worse so don't be overly concerned about them.
Off to the right of the picture, kinda looks like aiptasia tentacles? I'd make those a priority before the hydroids!