clear ones are harmless, well both really are. but the red ones will congregate on a coral and smother it, blocking the light from it. and of course if the coral doesnt get light it dies. the clear ones will die off on their own when their food supply runs out. they are known to hunt copepods, im not sure what else. thats reason enough for me to get rid of them! im not sure what the red ones eat, i would assume they were pod hunters as well.