Was machen?stamm hat flecken pilz?

that's mold. or Botrytis. You need to naturalize the fungal spore in that area by wrapping it up in some paper towel soaked in peroxide and then cut the branch about 2 inch below that wound. You can try and save it but chances are it will return and just spread everywhere. amputation is the best action IMO. You can use hot wax to seal the wound or just let it air dry. Given the mold is already fruiting/going to spore on the plant, the spores are in the tent and flying around. You will need to watch your RH like a hawk and keep it low in flower as you now have a increased risk of bud rot. The mold that you see is just the fruiting body it has thing like roots running into the plant. that's why you need to give it a wide birth and cut min 2" below where you see it. The spores are present everywhere at all times. its just if they have the right conditions to grow and reproduce. damp enviroments are a perosnal fav of theirs.