A clogged floor drain like this is a breeding ground for fruit flies.
That's a lot of debris.
Applying some Bio Drain to remove all fruit flies that are still in there.
Drink fountains that contain sweet beverages like juice or hot chocolate is a prime breeding ground for fruit flies.
Never let sticky liquids sit in a fountain tray for a long period of time. Those "bumps" in the liquid are actually fruit flies and chocolate powder chunks.
Make sure to empty AND sanitize the drinking trays as often as possible or else fruit flies will come.
Using cleaning solutions, like Hot Spot will both sanitize the tray and remove all fruit flies.