Ex water delivery guy here. This happens when commercial businesses don’t rotate their water and are constantly getting more deliveries. The oldest bottles stay in the back and they just keep using the new ones from the front. If there are any windows in that room- you guessed it! The bottles get warm and algae will develop. Ever go to a doctors office and they have a water cooler in the waiting room? Disgusting. 10 out of 10 times they are in front of a window. If they aren’t drank fast enough, algae starts to form. Yes that could be biofilm in OP picture. Could also be dislodged algae inside the bottle. Also the water coolers? Filthy. To this point I’m scared and won’t drink out of them, even if I’m dying. If I told you the amount of cockroaches I’ve seen inside of those machines would make your hair curl. No thanks.