LOL - it's a mesh media bag I put around the internal filter to keep the baby shrimp safe. It does kinda look like a sock tho, now that you mention it.
I don't think your tank is overplanted for the amount of shrimp you have right now. You might need to thin out the plants as they grow and take up your tank space. The reason I took out some of my plants is because my moss took over the entire bottom half of my tank. My fast growing plants were just too invassive and I believe they were out competing the microorganisms for nutrients. I really like the look of you tank
The emerald green of the plants really highlights your super red shrimp. I bet in the room it's in really catches your eye first, the perfect addition to any room. Kudos
They will self limit as long as you don't over feed. If I were you I'd be planning to gear up to at least a 10 gallon tank. Little tanks just have issues and they go out of control before you even notice. One day you think everything is fine, the next all the shrimp are dead.