it is undoubtably one of many small anemones which can come in on LR...its tough to tell if it will be a nuisance, my only advice would be if you notice it dividing/spreading rapidly to consider getting rid of it. as far as whether your clownfish will "swim" (we call that hosting) in it the answer is almost certainly no; right now it is undoubtably too small and i doubt that it will get large enough that it will