Whats this ?

That is most definitely bud rot, and it will spread... best to cut the infected part off with sterilized utensils and spray the place you cut it from with a 1% hydrogen peroxide solution. You're going to have to examine every single bud as well... look for small leaves which have completely died - this will indicate where more bud rot is - cut out ALL that you find and treat that area. If the problem is extensive enough, you may decide to harvest the whole plant. If the problem is NOT yet extensive, you need to get the humidity down to have a shot at controling it... and increase your air flow with some defoliation and more fans... left untreated and in those conditions, you may lose your entire harvest. Good luck! I've had to deal with a lot of bud rot this season in my outdoor grow and it's awful...