That's definitely bga. If it smells like a swamp, that's the clincher.

I had awful infestations of this stuff. It was caused by:

1. Lack of water changes with thorough gravel cleaning
2. The higher the light - the faster it would grow
3. No dosing of ferts

Once it set in, it was NOT cured (for me) by:

1. Increasing circulation
2. Reducing light (unless reduced to almost nothing)
3. Dosing anything
(tried it all, extensively)

Eventually, it WAS cured for me by:

1. Manual removal which in my case involved removing and replacing all the gravel, as it had stuck to it and was impossible to totally remove from each piece of gravel even if it was dead, plus removing all plants/leaves that were infested
2. Regular (at least 2x/week) 50% water changes with deep gravel vacs

Dosing ferts and reducing the light intensity prevented it from recurring, once it was almost gone.