Some kind of mold or mildew, I'd guess. Based on what I've read here and elsewhere, it's a fairly widespread and well documented problem with Diamond Shield.
John & Diane, fulltiming since '12 02 DS, FL, Cat330, '04 Element NHSO RVM103
John & Diane, Fulltimers. RVM103 NHSO
On the road since June '12 with Lincoln, the guard cat.
2002 Dutch Star 40, Freightliner, Cat 3126, 2004 Element