Personally, I interpreted this as blood all over Micheal Kahnwald's face indicating that Jonas was seeing a ghost/dead person/he wasn't real etc.
I saw the blood as also representing Jonas' fear of his father's death. It's mentioned a couple of times throughout the series that Jonas spent a few months in a Psychiatric hospital after his father's suicide, the bloodied visions of his father could be a 'physical' embodiment of the psychiatric issues that he was having.
I never noticed it on Jonas so couldn't comment on that.
That's just my interpretation, I could be completely wrong and it is just related to his father being a painter!