How old is the bird, how long have you had them?
First thing I would do is treat for coccidiosis, that is a common reason for weakness, lethargy, runny, watery, mucousy, or occasionally bloody (not always), droppings. It can do a lot of damage to their gut, and can progress to death quickly. It's a common protozoan type parasite. Treatment is Corid (amprolium), available at most feed stores, tractor supplies, rural king, etc. Images below. It's often in the cattle/livestock section. Many prefer the liquid for convenience, but either will work. Dosing is 1.5 tsp of the powder or 2 tsp of the liquid mixed in a gallon of drinking water (dosing on the package is for cows, chickens are different), it should be the only water it has access to during treatment. Treat for 5-7 days, make the solution fresh daily. If the bird is not drinking well on it's own, then give some of the mixture directly, orally every hour or so until you see the bird drinking on it's own. You can also give a straight oral dose once a day
in addition to the water mixture. Dose is by body weight, instructions here:
https://www.backyardchickens.com/threads/corid-oral-drench-instructions.1211991/
Corid is very safe, and will do no harm even if it was something else. I would start treatment right away, don't wait, the more damage that is done to the gut, the less likely recovery becomes.
