That looks like Botrytis (gray mold), and needs to be removed from the plant... soon. Destroy the section that's removed. It is easy to spread the spores, so turn off any fans in the room and remove the affected area of the bud. I'd cut it back a little further than the area, just to make sure it all gets removed.

High humidity is a common cause of Botrytis, so I'd see what your humidity is and make sure it stays under 40%. I like the RH to be even lower during flowering, but that's just me. I'm not sure if it would be a good idea to spray the plant with a fungicide or sulfur, so I'd hold off on that and increase ventilation/reduce humidity until someone can chime in on using sprays this late in flower.