Cherry shrimp have their days. Some days they will dart around the tank and some days they'll pick a spot and hang out. Just be aware a 6g tank will need a lot of water changes if your shrimp population explodes. I was 17 when I first kept cherry shrimp. I used a 5g tank. I let the population explode from around 10 to well over 50 and almost overnight I had a massive die off because of water quality. A small tank is always more work, just be ready for it. That being said, they will "clean" the tank scavenging for leftover food but they are still a bio-load.
Shrimp love moss and it does well under low light! Give the hair grass a chance before you get rid of it. It may do what you want it to do if you trim it once it starts to try and grow upwards. You could also try removing the lid and buy a cheap "clamp on" shop light. That would allow you to run enough light to carpet the hair grass.
"One generation plants the tree, the next generation gets the shade"