What is this?? Does the tank need to be broken down & restarted!!?

[TifNee837]

Not entirely sure what this is - so wasn't too sure which thing to post this in, to be completely honest!

These are pics from my best friends tank (her and I have grown together in this hobby - literally from the start of *my* obsession 4 years ago to now - and I'm at a total loss here)

 

First pic was from 7 weeks ago - a green algae can clearly be seen beginning to grow on the substrate and rock (rock = bottom right front corner).

Before the next pics - here is tank info & parameters and whatnot :: 
~ 45 gallon tank ~ has been running and cycled over a year now ~
•temp 76°F
•pH 7.6
•ammonia 0
•nitrite 0
•nitrate about 40
•phosphate btwn 0 - 0.25
•dGH 19°

Tank inhabitants : 
•10 or 13 Neon Tetras
•1 Molly
•3 Guppies
•1 Platy

Tank is fed every other day (sometimes 2 days in btwn)

15 - 25% weekly water changes (more if necessary)

Had started planting the tank before this mess, and had a few aponogeton plants, small piece of spider wood with java moss and anubias coffeefolia, and a bunch of small windelov java ferns, some small val plants and one or two other "root feeders" - so started to do less "deep" gravel vac'ing so the plants roots could expand and settle.

Well..here's the mess!

 

There was 2 weeks in the time between when the first pic was taken and now, when grav vac was not done at all but water changes still were done.

And the sponge filters were both/all taken out and cleaned a week after the first pic.

 

So, my fellow #nerms - what the HECK is going on!?

 

Help! PLEASE!!?

[Kirsten]

Wow that is wild! Did a child pour in some dry cement or something? Did a piece of plastic melt in contact with the heater and coat everything?

Other than that, my best guess is some kind of extremely funky mold or fungus. Like what might be growing under the lid of my compost bin if some bread dough got in there.

Does it smell? Is it hard or gloopy?

I wouldn't normally recommend a total tear down and restart, but this looks pretty extreme, and not knowing what it is means I have no idea how to treat it. 

Edited February 8, 2021 by Kirsten

[Aubrey]

Wild guess, but it looks similar to rock snot, or some kind of algae bloom that you might see in a river or stream.

[Squeegee79]

Perhaps a type of cyanobacteria similar to blue-green algae? 

[Calvin]

This might be helpful. 

https://www.aquariumcoop.com/blogs/aquarium/mulm

[MDoc]

This isn't mulm. Mulm of that degree would take faaaaar longer than a couple of weeks to accumulate. 

I agree that this looks more like a cyanobacteria or a fungus or rock snot like @Aubrey mentioned.

[Colu]

Never seen anything like that it looks like thick mud

[BlackLabelCarling]

I agree with other re cyanobacteria. Not to be gross, but has your friend smelled it? That is a good indicator for cyno. Or if she hasn't clean it up yet, test the nitrates again to see if they have lowered. 

It also looks like rock snot (didymo) but seems unlikely because it prefers cold water.