They are colonial hydroids. Although they will eventually self limit their population, they are very invasive and can sting other creatures. IME, the best way to bet rid of them is to remove the rock and scrape them off. Wear thick rubber gloves - the tubes will break off easily and end up in your hands sorta like splinters, except they are difficult to remove.
You can also cover them in kalk paste, but the tubes will be left behind, just waiting to jab you in the hand.