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.