Beneficial or harmfull fungus?

That's called mycelium bro. Its beneficial. It'll go after a few weeks. The plant will eat it up, as it decays it gives your plant a great boost in many areas. Fungus is generally your friend, unless you see it on your plant. You are growing in soil. You have a good thing going buddy. Now, some of it might go green, and look like mold. Even more so if you use any synthetic nutrients like a calmag. Try not to though. (As cal/mag in an organic grow is not good, especially with fungus. I can think of the name, but I know it'll cause issues. So if you can get an organic cal powder or just an organic cal and epsom salt. (If you have water above 170ppm you won't need calmag) but a dash of 250ppm Epsom salt never hurts during late flowering.. Good luck pal.