u/penisharvester

Hello, so my partner and I are panicking because after few months of moving in we decided to remove the bed cover and give our mattress a sit in the sun. However we were shocked when we found out that bottom part of the mattress is full of mold while the top part is all clean and I think the landlords were trying to hide it by flipping and covering it with a cover and we regret that we didn't check it earlier. Now we don't know if its fixable or if it's too late to clean it?

Also I don't know how it is possible since our house doesn't have any leaks, we never spilled anything on the bed, and our room is most of the time dry.