Excellenn-n-n-n-t - nice image

Those are "food"....Naturally occurring organisms which, left alone with baby fish would be eagerly accepted as food.....
They're related to the millions of naturally occurring copepods - nematodes, rotifers, mollusks and crustaceans that eventually end up living in and on the thousands of square feet of biofilm in the good, balanced ecosystem.

They are not a fish pathogen.

IF the fellow's fish are pinking up, that's a sign of stress - a closer look should be taken at:

1. ammonia
2. nitrite
3. nitrate
4. pH
5. Alkalinity

then

1. water temperature
2. crowding levels (more than one inch per ten gallons of water?)
3. new fish without quarantine
4. possible fish wet mount biopsy if none of the above give it away.