It is called a “vent” and it looks like it has pasty butt. You can carefully clean the area with a q-tip dipped in warm water, but be very careful not to rip the delicate skin around the vent.... you have to remove the poop completely to avoid a blockage; pasty butt can be deadly to chicks if it treated immediately.
Yes that should work. Maybe a little dawn dish soap as long as you thoroughly dry her, or make sure she goes right back under the heat lamp. Also watch the other chicks and make sure they don’t peck at her vent and cause injury. Chicks are very attracted to abnormalities and curiosity will cause them to peck.We cleaned the vent with a q tip and it looks like the vent it self is backed up. I might make a little warm bath tub for her to sit in for a while if the q tip isn't working