How do you prove that he knew about it?

Rule: Seller is only requried to disclose defects he knew.  If he did not know, he could not disclose.

 

Assumption: A reasonable person would not live in a moldy house.  

Fact: There are "经年积累的霉"

Argument/Defense: So if seller knew about the mold, why wouldn't Seller clean it? See assumption.  So, the fact of "经年积累的霉" would indicate that the seller did not know.

Next time, pick your words carefully.  Just say "large area of mold covering 1/3 of the floor", don't speculate how long it was there. Otherwise you are working against yourself.