Slimy black goo

Posted this the other day but it's disappeared. Dismanted my DE filter for cleaning the other day and, after removing the top half of the tank, I found a mass of slimy black jelly-like goo(mold?) on top of the grid manifold. No evidence of it anywhere else. Has anyone else seen anything like this before? I've always assumed that water under 20psi would go everywhere inside the filter tank and not allow something like this to grow. The filter has performed beautifully and I never found this when I've cleaned it before. The only new variable is the switch from DE to powdered cellulose, which I like very much. Comments?
Hal