What could this be? 🤔

Pen • 8/17 🌈👧🏼 | 11/19 🌈👦🏼| 👨‍👩‍👧‍👦

While changing my son I found this weird mark on his heel/ankle area. I figured it might be poop so I tried to wipe it off but it didn't come off, so I thought maybe a bruise? But then I put his sock back on and saw there's color on his sock where the mark is, and bruises don't do that. The heck is this?? Ideas?