What? Algae? Bacteria?

[Gorsim]

I was sick and generally ignored a tank in the basement (nerites but no fish, filtered, lights on a timer) for a couple of months. Algae (?) was so bad I decided to "let it ride" for another month. Now this! Algae? Bacteria? Plant rot (mostly on the buce)? It looks like seaweed and has a similar consistency, kind of like firm jello. It's so bad it's interesting but time to clean it up. 40 years of keeping and I've never see this.

Thoughts please. Treatment?

Edited May 17 by Gorsim

[Guppysnail]

Your photo did not come through 

[nabokovfan87]

@Gorsim photo isn't working.  😞  Please try to reattach it.

Might be a type of freshwater sponge.

[Gorsim]

Photo reposted... Thanks for letting me know it didn't come through.

 

[nabokovfan87]

I am unsure what is going on there. More photos would help, it's a very strange thing. It looks like dead plants to me.

You'd want to try manual removal as much as you can. Netting out all the plants to be able to clean them and then siphoning the tank, etc.  Reset everything so to speak and check for dead snails and other ammonia sources.

Light added to a timer is also necessary if that hasn't been done already.

[Galabar]

Water parameters?

[Guppysnail]

That looks like biofilm but could be a lot of things. 
Does it smell. Pull some out and rub it. If it is a nasty smell it may be Cyanobacteria or similar starting to grow around the algae maybe. If there is no smell I would trim up all the dying plant leaves and remove as much as possible. Start doing regular maintenance and see if it clears up on its own.