Looks suspiciously like the characteristic 'V' of mange mites, your vet will probably recommend a strong Ivermectin product - injection or more commonly a spot on such as Xeno450
Xeno Spot On (vet should weigh your guineas for a dosage guide for the correct strength product).
Your vet may do a skin scraping & look under the microscope to determine if it is mange mites.
Ps.. Vets usually put a big mark-up on Xeno products, so just buy a pipette for each guinea for the initial treatment, then buy a box (6 pipettes?) online as it's so much cheaper!
The Animed link above has quite competitive prices. X
I expect your guinea will scratch & bite himself more in the first few days of treatment if it is mange mites, as the mites bury themselves under the skin, they cause more irritation before they die off.
You will probably need 2-3 treatments approx 7 days apart. This will kill off any juveniles that hatch from eggs in that period.
Both your guineas will need treatment as the mites can transfer from one to the other, although companions don't always seem to have symptoms.
Give their hutch/cage a thorough clean, hot wash any fabric bedding/toys.
If you have any wooden bridges, etc. just pour some boiling water/hot animal safe soapy water over wood to be on the safe side & leave to dry thoroughly.
Although the mites generally live & feed directly off of the guinea's so it's not essential but more for peace of mind, ruducing the odd stragglers
![smile :) :)](https://cdn.jsdelivr.net/joypixels/assets/8.0/png/unicode/64/1f60a.png)
Completely replace all hay in their hutch/cage for unused/fresh hay.
Have a vet check them sooner rather than later, just because mites cause painful itchiness & if left too long can cause other problems. The sooner treatment starts the quicker your guineas will be more comfortable and recover.
Good luck, let us know how you get on xx