Poultry Talk Ontario (pto.ca)

Chick is eating and drinking well - still in the "brooder" (dog kennel) in our moderately cool garage (it gets some a/c off the house). Just noticed today she/he has this y rear. Does not appear to be clogging up her vent. Should I be concerned? Action to take?

I would just gently wipe her clean with a moist tissue because it will build up and maybe cause a problem later.

I had two chicks with y butts this year, and I used warm water and those disposable cotton makeup remover pads (really inexpensive for a roll of them at Dollarama). Just wet a pad in warm water, gently work it loose and remove it. Once done, you can dab the area with a couple more pads to dry it a bit.
If I were you, I would try to remove it, because manure build up in that area has a habit of turning into a big ole vent blocking clump of cement in a hurry once there isn't enough clearance for a direct ground hit over the stuff that's already clinging to the butt fluff. If your chick isn't dust bathing yet, it has no way of getting rid of it, and it will just get bigger and harder to remove.

When I had 2 chicks with y butt I use witch hazel diluted to cleans them up and I also used makeup pads. The I added apple cider vinegar to their water. No more y butt.