The hermits will be Clibanarius digueti. They are pretty adaptable as CUC animals go, but they are still omnivores. CUC hermit species like C. digueti should not be relied on for algae control - that is another hobby myth. They are primarily omnivores and carnivorous scavengers and need a diet along those lines to behave sanely long-term. They will pick at algae, but often they won't actually eat much of it and will just tear it up while looking for other things. Attempts to force them into unnatural amounts of herbivory often end with them eating something they weren't supposed to after a while (snails, corals, etc. - main cause of "my crab ate my healthy coral" complaints). Hermits and true crabs will also go after other Crustaceans when they are vulnerable right after a molt, so that could well be where the emeralds went. Proper grazing animals like snails are better for algae control.
I seem to recall they got sluggish and developed a milky tinge to their color the week leading up to the death.
Old individuals will start to slow down and sometimes go white before death. Going white rapidly without any slow-down can be indicative of a molting problem, but the slow-down sounds to me like they might have been old. Sometimes it starts in one part, like the tail, and spreads over a couple of weeks. Cleaner species don't live a massively long time; usually only around 3 years or so, and they will be some amount into that when you get them - no telling how much when wild caught. Did you see any successful molts from the shrimp before the slow-down and color change? If not, then something odd is definitely afoot.
A pretty certain way to make sure it's not dietary iodine that's the issue is to feed shrimp or krill in some form regularly. Frozen or fresh is great, but it can be messy and a pain. Pellets also work. This is the type of pellets I use:
http
/www.omegasea.net/products/nutrition/shrimp-pellets
I feed all of my tanks with Cruastaceans the pellets daily. For species tanks where the isn't really a CUC, that's the main part of the diet. In tanks with a dedicated CUC, it's more of a supplement so that it's not enough to cause the animals not to eat the uneaten flakes and such, but it's enough to ensure that if they are desperate for that food they certainly have access to it. When doing that, you have to play the amount by ear a bit since the demand will vary based on where they are in their molting cycle. If a war breaks out over a few pellets then it's a cue that you need to add a few more so they don't kill each other over it. Hermits going totally bonkers over something is usually an indicator that they need something from it. If you see some half-eaten pellets left sitting about while the hermits are picking at other things, don't add any more for a few of days until they clean it up.
Feeding CUC animals like I do is considered controversial among some of the larger reefing community, but the fact is that many tanks experience unnecessary invert attrition or require more frequent than necessary "CUC top-ups" due to failure to support those animals well. It really takes quite a large and diverse tank to have a CUC sustained long-term purely on algae and and uneaten fish food while keeping the tank clean.
