My suggestion is, first, provide more info on the part in question; I can only assume (based on the pic) that's a mechanical caliper...
The best advice I can offer is rust or dirt/crud has, most likely, built up on one of the many moving parts; disassemble the entire caliper, clean everything thoroughly & reassemble using a smear of grease on the moving parts (this will not only provide lubrication but, it will also prevent future rust)...