if they do the rubbing only at night it is probably a behavior thing.
fish live with parasites, due to crowding or when water quality is poor it weakens the fish and the parasites are opportunist. if you are concerned about the fish having parasites you can look at a sample of their skin mucus under a microscope. there are several parasites that will cause the irritation that makes them rub.
if you have a microscope you can gently net the fish and gently scrape slim from the scales around the gills and look at the scraping under the microscope. if you have parasites you will see them. you can Google fish parasites to identify them. problem is the meds toxic enough to control them can not be used on food fish. don't waste your time with salt. african cichlids are very tough fish, unlike many other species of fish and normally don't die as a result.