My tank is technically a non-smoking zone, but I think something is putting its butts out on my cabbage leather.

This guy waxes and wanes with extension, full "leaves", color, etc., so it's not unusual to see it look "unhappy". Lately it has been in a funk. The other day I noticed a black spot in the middle... That black spot has tripled in size in just a few days. A picture is attached that looks a lot like as aquatic cigarette burn.

What could be causing this? A worm? Another pest? Any thought on whether I should be worried? Doing something?