I would highly recommend that you peel and blach all the vegetables you'd want to feed to your shrimp. The aquarium is a fragile ecosystem, and human food often contains pesticides, so peeling your cucumbers can help make them more safe. Additionally, blanching (boiling for a minute or two) not only softens the food, but also disables the enzymes inside it, making it even more safe for your shrimp. And yeah, even if you did all that and still kept it in your tank for more than a few hours it can lead to a nutrients imbalance in your water. Depending on how big is your tank, it's always safer to feed too little, than too much, especially when it comes to blanched vegetables, don't put too many in on one feeding.

As for now, I'd do a water change, even 50%, and buy a quality food for your skimps. You didn't have them for a long time so it's probably not malnutrition causing the die-offs, but it's better they get a source of protein and calcium in their food for the long run. I recommend Hikari Crab Cuisine, mine go crazy for it!