I am far from a tilapia expect, I think Rainman or Weissguy would qualify as that, but if I recall correctly... Tilapia hold their eggs and young fry in their mouths. If what ever you are seeing is all over them, I don't think it is eggs or fry. It sounds like some may have been handled a bit rough and got a little stressed and got some sort of fungal infection. That is just a guess though... A picture would help greatly.