Bud rot is mold that grows from the inside of buds that have too much moisture. Not enough light will cause branches to stretch, and will result in underdeveloped flowers. Basically, you'll have a plant with a bunch of larf.
Just a high relative humidity will be enough to Foster bud rot. It does look a bit like that... Pull the bud apart and you'll know straight away... You can cut away the rot and harvest the rest but imo plants with botritys aren't worth smoking, I'd separate the rot and flush it then use the rest for edibles
Looks to me like it started flowering and then stopped flowering and went back to veg, then all the 'bud' that was there finished up and died or rotted as was suggested.