I found one just like that a few days ago and removed it imediately. I may have more, because I noticed a difference in my long piece of rock with my healthiest zoos. One half of it looks like it's dying off. I just gave it a very thorough going-over in the bright sun and found nothing. I even broke a dense part of it away to further examine it and - nothing. Now it's in a different tank with no other zoos - just in case.