Baby chicks have diarrhea
Chicks, and chickens, make several different kinds of poops and some of them are just a dark brown, more liquidy type, these are also much stinkier! These are normal.
With chicks probably the number one thing to be aware of and just watch for is coccidiosis. It's very common. Chicks will go from completely normal to suddenly lethargic, standing around hunched or puffed up, not wanting to eat or drink. It's a very good idea for anyone raising chicks to keep Corid, either powder or liquid, on hand to treat just in case it flares up. TIme is of the essence and you don't want to have to be searching around town for it when chicks are sick.
Other then that, you can watch for pasty butt too. If you see that just gently clean it off with warm water. You can put a little vaseline or olive oil on the area to prevent further sticking. A teaspoon per quart of water of raw apple cider vinegar (such as Bragg's) does wonders preventing pasty butt. Check that your brooder temps are too warm, that can cause dehydration and result in pasty butt.