When you're having issues, get some pictures to document what's happening.
Mucus/white fluff can be a whole assortment of things.
- Why only 1 otto cat? Was it to combat the algae?
- Why 2 mystery snails?
- How long has this tank been set up?
- Also make sure you shake the hell out of the test solution bottles for a good 30 seconds before you add testing solution to the glass tubes.
- Does your ammonia test solution require 8 drops from 1 or 2 bottles?
I doubt the tetras are generating all that peckled mess on the substrate. I think the culprets are actually the MYSTERY snails. A single mystery snail depending on it's size can produce as much bio-waste as I believe 3 neon tetras.
Feel free to correct me if I'm wrong on that.
Photos of your current setup & close ups of the problem areas would be helpful.