Molt problems

[The Grym Reaper]

I've also heard from a few people it could be something called a mirror patch? I've done a little research and is there a way to be able to tell if it's this or a bit of molt.

I don't think they have a visible mirror patch as they get larger (my girl is only around 2" and the blue form so someone correct me if I'm wrong) which is why I think it's a bit of the old exo, on the right hand side of the patch in the photo it looks raised slightly in such a way that would indicate so but you'd probably be able to tell better in person.