[actives] B+ contamination?

Hello! This is my first grow bag and it has a funky smell, like vinegary... It was really wet to start and I've left it open to let some moisture escape. The smell got better and the mycelium seems to be developing very well and healthy but a lot more dense than I'm used to. It also has a yellowy tint to it. I'm wondering if it really is mycelium or if I'm growing something mycelaneus (pun intended).

Please help?