My iguana also had that crusty black stuff after a long battle with
mites.
Take a good look for them in his/her cage and on his/her.
It's pretty much an infection. Your iguana should be able to fight it off no problem. Soak him/her in water mixed with Betadine (i think?) or benedine. One of them or close to it anyway. Soaking in regular water just helped mine because that helps with shedding and once the source of the infection is gone, then it should begin to go. Scarring occurred though but that is shed too in a while.
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