Good evening,
I need your help. A few days ago my peace lily started to develop a growth, in not sure if it might be a fungus or a burn but I am concerned since it started to spread on the other flowers and leaves. I usually water them once a week and I'm sure what it could be. I have attached pictures of the growth. Please let me know what it could be and thank you.

I assume you are referring to the black tip on the flower's spadix. It is not a disease. It is probably normal dieback of the flower as it ages. Eventually, the entire flower will turn brown and can be cut off.

Improper watering can cause premature flower die back. Try to water your Peace Lily just before the leaves start to wilt.