Looks like runny, bloody poop to me. That's one of the signs of coccidia. Are any of them lethargic, and puffed out? Are they all eating and drinking normally?
I would treat them with Corid or amprollium for coccidiosis. Signs are diarrhea or blood in droppings, lethargy, standing around puffed up or hunched, poor appetite, then later, refusal to drink. Dosage is 2 tsp. of the liquid, or 1.5 tsp of the powder Corid per gallon of water for 5-7 days, changing water daily.