Weird green fuzz on betta fish fins

[Stormcloudferret]

I NEED HELP IDENTIFYING THIS 

edit #2: He died last night. :((((

hi y’all, I recently noticed my betta fish was very lethargic, only sticking to the bottom or the sides of his tank and wasn’t moving. He also hasn’t been eating recently which is why he looks so thin in the provided image. Upon further inspection I noticed a fair sized clump of what almost looks to be algae growing on his tail. I’ve searched the web but haven’t been able to find anything that seems accurate to his condition. He won’t eat anything, even when I offered him some frozen blood worms (something I only give him as a treat). He went longer than usual without a tank cleaning (school, life obligations and I was sick myself for a while) and therefore there was a fair amount of algae growing. He lives alone in a five gallon tank with one fair sized decoration (it has a natural stone look with lots of holes he can swim in and out of and places he can hide in) and one small live plant (I got it a long time ago and have since forgotten what kind it was, but I do know it was recommended to me by an aquarium co-op employee). I also have a heater (came with the tank in a kit) and an aquarium co-op sponge filter (meant for a 10g tank, but it’s an adequate amount of filtration and flow for my 5g). Does anyone know what this could be? I’m hoping to identify and cure it as soon as possible. I just cleaned his tank and gave him an around 80% water change. Thanks.

Edited December 21, 2021 by Stormcloudferret
Photo didn’t come through the first time

[laritheloud]

Hi there, the picture didn't come through. Can you try uploading again?

[xXInkedPhoenixX]

@Stormcloudferret I'm so sorry for your loss. I'm not sure what it is exactly but possible fungus. One of our other members has a green growth on an Oto. I'd previously not seen anything like it. Treatment is still in progress there, not sure if @Streetwise has started Ichx or seen any improvements yet. Here is that thread, maybe you'll see a similarity in what your poor betta had.

Otocinclus Problem