My thought is that it's exuding some dead cells. If you "waft" (I don't know if that's a word, but I like it.
) your hand over it, does it come off as dust, or slime, or do you have to get more aggressive to remove it? Is there a build up of loose detritus in the center of the "bowl"? If so, take a few good wafts or a powerhead to blow it clean. Circulation, like a nice breeze, does wonders.
Do you add any iodine?
Hows the Mg level, as well as Alk and Ca in the tank?
Do you like long walks on the beach? Ooops, sorry, wrong question,

Edited by Mark Peterson - June 15 2006 at 3:15am