Should I be held responsible?

A couple months ago I had reported to my apartment that were some brownish stuff on my wall and they had repainted it white. However, a couple months later I realize it might have been mildew because bluish-greenish stuff grew from the same section of the wall. I was told by the property manager that the apartments are prone to mildew because of their construction and that while I was not responsible for actually causing the damage that I am responsible because according to their opinion I did not report it in time.