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Its not a scab, its a mirror patch. Its a patch of urticating bristles that's reflects light differently than the bristles around it. A few different genera have them.
Not to sound rude in any way, but did you do any form of research before acquiring this animal? Urticating hairs are mentioned in literally every book ever written on tarantulas. The reason I'm asking this is because this is extremely basic stuff. Even someone completely new to the hobby should know things like this.This is something that they do as a defense mechanism. Many species will lay down hairs on their webbing or on the ground near their hide to deter predators. If you're handling it I'd suggest not doing that, as this will give it more of a reason to do this. Again not trying to talk down to you or anything, but I'm pretty shocked that you are so surprised by this feature/ability that they have.