I'd spray mold retardant and scrub away until it stops coming back, i f its not dry rot. You may also have a leak in your roof with water directly overhead or running down a rafter somewhere leading to that beam. Check your attic well, or have a roofer look at it for a free estimate of repair. Good luck and god bless. : )
I'm just a guest on the site but happened to see this. You should get the mushrooms checked in case they are dry rot - sorry, don't want to scare you, but dry rot does produce things that look like a cross between a mushroom and a pancake. good luck.
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