A clear indication of caterpillar damage. Cut off the offending flower cluster and dissect it you will find caterpillar poop, you may need to dig deep to find the worm. Let the rest of the plant finish as usual. The trick is to observe your plants and catch it early. Cut the bad stuff off and let her finish. For caterpillars, I have used BT up until 2 weeks before harvest with no ill effects and do a 3 step wash before drying.
Healthy G13 about 5 weeks into flower.
Same plant but this flower looks to be noticeably browner. I let it go for 2 more days this top died. I snipped it off and sure enough, I found the worm.
She was harvested on Sept 14 and dried to 7oz of smokeable flower and 3 oz of trim.