Dead snail maybe?
Warr, just a thought. There seems to be air bubbles in your subtrate, if it is it could be bacteria growth.
Warr,
I think it may just be an anoxic area.
Anoxic refers to an oxygen depleted area. This also occurs naturally in the ocean.
The bubbles normally occur when you add the water after the substate is in place. They become starved of oxygen and then they can feed bacteria like cyano.
Heres a nice explanation on Wiki regarding sea water,
. Anoxic sea water - Wikipedia, the free encyclopedia