I don't normally advocate medicating unless absolutely necessary but if he is doing it on a somewhat regular basis I would just nuke him with some coppersafe. It is really safe stuff and should knock out just about any creepy crawly. Because there are external parasites out there that you can't physically see.
Plus it's really easy to dose.
It says it isn't dangerous to fully-shelled inverts but I would remove them to be on the safe side.
Flicking can indicate many parasites apart from ich... you could be looking at early velvet, trichodina or other protozoan parasites. The fact that all of your fish are doing it despite okay water parameters also makes me suspect something 'contagious' like a parasite.
copersafe ABSOLUTE no for any inverts, especially shrimp. does the fish(es) in wuestion do this alot? every so often a fish gets and itch, and thats how they scratch. if it is frequent, females or no in his view, id be on the lookout for ich firstly. will look like grains of salt on the fish. easily treatable and very good survival rate. if this continues and NO sign of Ich, take a flashlight and with the tank lights and room lights off shine it on the fish(es) in question and see if they have a dusting of what looks like rust on them. that is velvet and calls for a totally different treatment.
let us know.
cheers