What is wrong and how can I help?

Noticed one of my 3yo girls was standing alone (near the house, I assume looking for me for help), and I went to go see her, and noticed this on her vent. That’s a dead leaf attached to the poop/mucus.

I have an area to separate her and am going to clean her up, but what else can I do and what could the issue be?

Update: added photo to comments after cleaning her. We just got back from a week-long vacation to visit family, and I think it’s a prolapse (?) that festered? I’m not sure what to do for her now or how (or if) I can help.