Sounds a little like pastey butt to me. That can be caused by too much heat. But I thought that happened mostly 1st 3 days.and my little chick that had that had vents that looked similar to yours after washing.
I have an older chick 6 weeks who's vent looks just like this a little swollen, red, missing feathers and white running down its bum. He has been quarantined and I have been trying all day to figure out what is wrong. chick seems otherwise healthy no signs of trouble with the rest of the flock...