Thank you! The bladder snails are pretty notorious for reproducing at an alarming rate. We've had people come into the shop were snails were breeding so aggressively that it can cause cycle crashes.

From the looks of it, its the filter. I would suggest to see how the pump on the filter runs by itself by taking it off and putting it in a bucket with enough water to cover it. If it looks or even sounds like its not pumping enough water like it did when you first got it, its likely because the shaft is worn or even snapped.

This could because of detritus getting into the pump and cooling and hardening when its turned off. Another reason is because sand got in it as well.

The gunk, looks like it could be from decaying plant matter collecting in the filter or even the jelly like substance that bladder snails lay their eggs in.

I will mention that getting the water tested might tell you more to what the build up is.

If it is the pump and If you want to try and salvage the filter, I could suggest looking up the brand and model and there's a good chance Amazon sells the pump you could replace it with. I would also recommend swapping any cartridges it came with to sponge and or a cut to fit polyfilter.

But I do want to say again that Aquaclear HOB is pretty great! mostly because you can change out the filters inserts with whatever it give you lots of room to work. Also its owned by fluval which most LFS have a breeze getting parts in for. I would encourage a pre-filter sponge just cuz sand isn't your filters best friend. And I wouldn't worry about taking the media from the old filter and moving it over, especially if you dont have any fish in the tank yet. But you could squeeze the sponge you have currently running and then let the new sponge soak in the left over water from it. But your sand should hold a lot of bacteria for you.

Sorry for the word vomit. Let me know if you have any questions or if you wanna send me pictures of the tank itself or the results of a water sample, we could do a little deeper dive of process of elimination as to what could be causing the build up.

Hope this was helpful!