Ok, I really not sure where to put this..

I have Birkenstocks.. Sandals.

Well, I just sort of moved to my Bf's house 2 months ago. And I helped mow the lawn maybe 4 weeks ago. Anyway, I mow'd in my sandals.
I left them at the bottom of the basement before you walk up the stairs from the garage.

I have had my sandals forever... They have been wet before..

Well, they got kicked under the wooden steps.. and today I went to wear them and one had mold all over the cork you stand on. Its gross and they are ruined!!

Why would one get moldy? The other wasent under the steps.
Is it the house?

It's the cut grass/grass juice. You might try vinegar and a toothbrush.

I have mow'd before... was it cause I didn't wear them for so long?

That's a possibility, but remember, if it's real cork, it's a natural product (from tree trunks). Maybe the organic nature of it couldn't take any more wet...