Is it normal for fish to rub against rocks?
Fish will get the occasional itch and seeing a fish flick against a rock isn't necessarily a cause for alarm. But if it's performed with any regularity, it's worth checking for ammonia or nitrite and chlorine in untreated tap water, as these can cause irritation.
Other sources of the same symptoms include particulates in the water from unwashed sand or detritus and more seriously, flukes or white spot parasites. As with most things fish-related, a water test will rule out a whole host of problems.