Not sure what it is, it could be a sponge or some organic residue that's resolving over time. However, you really don't need to run lights in a cycling tank. It will just fuel algae growth.
If you're really worried about it, just take it out and use a clean brush of some sort to get it off.
"The simplest explanation for some phenomenon is more likely to be accurate than more complicated explanations." -Occam's razor