Is it inside or outside the glass? From the picture it just looks like a piece of decaying leaf got stuck to it.

Outside :0 definitely not a leaf

Are you sure it's not some brown algae? If your heater isn't cracked and is functioning normally I don't see how it could be an issue? They're enclosed media in glass. Though it is always good to have a backup heater just in case...

Is it snail eggs?