I Normally Move My Fingers Around And My Veins Pop

I normally move my fingers around and my veins pop up. I know my veins pop up because I can feel them but I do this and look and there is this yellow stuff on my had. I don't know where it comes from, this has only happened 2 or 3 times now. I just scratch it off and I'm fine but the last time I did I dont know if it did this but now I have 2 small scabs where the yellow stuff was.