mosquito fish/ molly hybrids?? I never have myself but while netting in a super secret ditch, I caught a few silver mollies with black mosquito fish dots on there abdomen, they also had mosquito fish features... hybrid or not?
The dots could possibly be water flea larvue. I have seen them many times here on wild caught Gambusia. It looks normal for a few weeks and then the dots grow and then disappear. I suspect once the water flea hatches in aquarium that the fish eat them. Since it is in a inclosed body of water.