I am new to marine aquariums, I recently purchased 50kg of living rock for my tank, it has been in for about 3 week and I have noticed a caterpillar type animal living in a hole in one of the rocks, it has two little white eyes on stalks and white stripes on its body, it also has 2 rear fins on its tail but does not swim , it crawls along. Any ideas what it is anyone ??