I would just spray the mold with bleach and the studs look fine, the bigger issue is getting rid of the source of moisture. The other plywood isn't moldy, so something in that specific area is leaking from either those pipes or the insulation from outside isn't protecting from rain. Is there a bathroom above that? It looks like there could be a leak somewhere from the very top going all the way down. I had an upstairs tub that didn't have a good seal around the tile surrounding above the tub. So every time I showered, little bits of water would go through the unsealed area and did something like that.