m0useanswered grow question 10 months ago does it rub off?
Might be root sites forming if the site got injured. or scar tissue.
Keep an eye on it, if its the only site and its not spreading I would not worry to much about it. Could also try to isolate it with some medical tape. or cloth try and stop the spread of any potential spores. Maybe bush it with some peroxide 3%.
Good Luck!