Tenant complain about mold in the house in an area under the window where
some faint black spots about the area of one square foot each were on the
surface on the sheetrock. Washed with bleach but the spots reappeared. I'm
pretty sure this is mold.

I then remove the sheetrock to inspect the inside wall area. This wall
cavity looks dry but I'm sure there was some water intrusion from the old
aluminum window above. There are some faint pinkish or reddish blotches on
the other side of the sheetrock. I peel the sheetrock paper off on the back
side and the pinkish/reddish color did not penetrate through. Is this
pinkish/reddish stain mold or some funky sheetrock used in the old days?
There are no water spots. I don't want to rip out the whole wall if its not
mold. Sheetrock at lease 35 years old. Any ideas?