It looks like a permanent marker of sorts which can sometimes be removed by ethanol/rubbing alcohol. If it's been washed a lot, whoever, it'll probably be impossible to remove as the dye binds with the fabric which, it seems, is the case here, unfortunately.
I would either remove the fabric altogether and sew on a new patch or, alternatively, get another, darker permanent marker and paint all over the patch. Leave to dry, wash by hand a few times and it should stay forever.