Not sure that your scarlet shrimp is acting abnormally. Mine does not do a thing except hang around. Never once has "cleaned" a fish. Try regular cleaner shrimp for that.
Adjust your salinity slowly and see if it makes a difference for your scarlet. Don't expect alot out of him though. They are mostly reef decorations and serve no real valuable function except pleasure to our eyeballs. The reef will be happier with a higher salinity, so you have nothing to lose and everything to gain as long as you adjust it SLOWLY. A rapid change in anything usually spells problems and/or death to livestock.
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