P.S. My pasty-butt chick all grown up:
Unless something changes or the runny poop is due to a problem, time and observation is probably all that's necessary.
The only thing I put on mine was a little olive oil to keep poop from sticking.
I let nature do the healing for fear of using a medication that would make him -- or the others -- sick if they preened/pecked at it.
I've only had one serious case of pasty butt and that's more or less what the poor, irritated, little backside looked like.
It took him a couple weeks for all the swelling and irritation to subside and I kept and eye on him to make sure he wasn't being picked on because of it.