You may also try using small curved scissors if you have enough room (ie if attached by just the fluff and you can snip it off carefully). I wouldn't wait another week and risk these becoming blockages, especially with freezing temperatures helping more poop stick onto the existing pieces.
https://www.walmart.com/ip/Fiskars-4-Orange-Curved-Scissors-1-Each/34932916
If that won't work...
Bring the chick inside for this... Have the following things ready: 1. get the water flowing so it will run warm for you to rinse the chick butt so it's ready to go right away, 2. dry high-quality toilet paper, 3. dry washcloth, 4. hair blow dryer
Once the piece is removed, use the high quality toilet paper to get as much moisture off the chick as possible. Toilet paper works really well for this, much better than towels or washcloths, and is gentler on their skin.
Once the chick is as dry as possible using toilet paper, wrap the chick loosely in the dry washcloth and use the blow dryer on its lowest setting (use you wrist to check for how hot that is and adjust accordingly before using it on the chick) and blow-dry the chick until it's back to fluffy.
Note:
Chicks take this treatment a lot better if they have somewhere to hide their heads- especially ones who are broody raised and not used to being handled.
I use a long sleeve shirt for the entire process.
When I go to wash the chick's butt, I pick them up like you would pick up your computer mouse. The head faces your wrist/sleeve opening, the butt is by your fingers. Pick it up with your non-dominant hand, leaving your other hand free to do the work.
Let the chick tuck its head into the sleeve of your long sleeved shirt. They are MUCH calmer that way and generally speaking, only their butt gets wet, cutting drying time considerably.
The drying process (especially blow drying) can be done the same way - or on the counter loosely covered with a dry washcloth to mimic being under the hen.
Keep the chick (now dry) chick protected with the dry washcloth when you take it back outside to tuck under momma hen.