if you see this pest under water, try to move it to swallow water and you kill it as they stand still. if you see it on open sea, go to your raft and move the cnidaria close to your raft and you kill it. but if you already on deeper water, go near a wild water creature and us it as cover. but if there is no wild creatures, try find high rock or try slip your body on rock and you kill the cnidarians, but if there nothing and just you and cnidarians. well nothing to do unless u bring your megalodon or other water creatures that you tame 😁