Get rid of it ASAP - and you're going to have to check and re-check and re-re-check your other 6 plants daily... bud rot spreads by spores that are basically unseen so if you have a fan in your grow space (I assume, like all good growers, you do), they'll have flown everywhere... actually, even if you DON'T have a fan! Nasty shit... DO NOT SMOKE this! It can cause terrible lung problems. If you manage to get any of your other girls to harvest, you'll STILL need to inspect them as you're drying/trimming etc... You CAN try to cut it out with sterilized utensils and then spray the cut with a 2% alcohol solution.. but in my experience (and BOY do I have experience), this did little to slow the progress of the rot.
SO sorry.... I lost my entire summer grow to this nasty fungus...