weird spot

I found this on his underside. He doesn’t seem to be in pain but it’s the first time I’ve seen this. I have no idea what this is or what I should do. Please help out a worried owner!

Hey Guys I just joined and I'm nervous because about 1-2 weeks ago my guinea pig has some weird spots on his face and above his eye. I couldn't find what this was so I joined this hoping maybe one of you guys had an idea. I just got my guinea pig about a month ago and I've never had a guinea pig...