That looks like the start of fungal, likely ringworm to me; but please be aware that you need a hands-on vet examination as we can only guess.
Be aware that ringworm is highly contagious not just to other guinea pigs but also humans. Make sure that you follow the hygiene tips in our guide to the letter in order to get on top of it and prevent it from returning. There are no shortcuts; the invisibly tiny spores are shed in their thousands, get everywhere and can stay live for up to 2 years to cause another outbreak.
Please take the time to read this guide here. It also contains pictures of how a ringworm outbreak typically runs: Ringworm: Hygiene And Pictures
Personally, I would just leave the wound be as it is already scabbed over anyway and see a vet as soon as possible. Saline works on open wounds. It is diluted salt and doesn't sting badly, but it is to be applied to open wounds.