If you find lice (they are large enough to see- about the size of a flea, pinkish/greyish, fast moving, scuttling around the feathers above and below the vent), or eggs- (clumps of grey small balls at the base of feathers in the area)- you need to kill them with an insecticide. You can use dusts such as sevin/carbaryl or pyrethrin poultry dust- applied to the vent area more of the bird, repeated every 2 weeks or so. Remove any feathers (cut or pluck) that have clumps of lice eggs. There are other non insecticide treatments (DE, garlic ect)- I don't personally think they work, but some swear by them- if you like alternative treatments- look those up and give them try if you want. There are also many people that use off label systemic meds like ivermectin, it works, but the withholding time for layers is not published, so therefore it is a no no if your chickens or their products are used anywhere beyond your own table.
If you find runny poop- see if it is still there after a few good days on a good quality layer ration, consider probiotics ect. If it is still there (do you know what normal chicken poop looks like?), then you can either get a diagnosis of a parasite (or not) by taking a fresh sample to a vet who will work with poultry or take a guess and try to treat with meds from the local feed store.
Jess