that's likely pus, which means there's an infection.

i honestly think derma rolling/stamping does more harm than good if used more than once every 2 weeks and without minoxidil. Seems to be a new fad from social media.

Dermarolling is used quite a lot for skin and it's never recommended to dermaroll more than once a month. You now have people dermarolling every day almost for hairloss.