Oh dear that dont sound good.
Are you sure your hygrometer is working correctly.
If you have have moisture raining down that suggests high humidity and not enough airflow.
The brown stufff is budrot and is very contagious.
You need to check the rest dear.
It always starts from the stem and works its way out so its usually too late by the time you spot it.
Hope it aint to bad fella.Try to cover the buds with bags when removing so as not to spread it around.