Mildew at least. Is your phone old? I recently found mine in the bottom of my bicycle pannier where just a bit of moisture caused it to look like this. Wiping it off with bleach water and it was good as new. Still works even though my prepay plan has now expired.
I don't use phones very often as I prefer email where one has time to think about what one is saying.
I mostly use the cellphone while traveling long distance by bicycle. Have been riding around the local area, but not using the phone. Before this summer's traveling season, I plan to reactivate the prepay plan, or possibly get a Smartphone. Besides the phone, I also bring a netbook computer for WIFI access to the internet from stops along the way.