I agree, Brachys love to kick hairs. Hobo you beat me to it (experience is a good thing ).Doesn't look anything like mold. That's urticating hair.
I certainly wouldn't be puttin' "gentelmens venitians" solution on it neither!
It's not mold, or a bald spot.That looks very questinable indeed and i dont think its a bold spot from U-hairs being cast off. It looks like what Tarantula keepers guide describes as mold.
It says to swab the infected area with "a 1 percent gentian violet solution." But be careful, it stains clothes and is also carcinogenic.
so yea i would definatly look into it.