WATER drinkers should be regularly cleaned to avoid bedding and dirt building up in them: some horses will not drink out of an unclean drinker. Most drinkers have a drain at the bottom for easy water emptying.
The slime-busting weapon of choice is any stiff brush. Using an apple cider vinegar wash is a good, natural idea to rinse the walls of the trough. You can use a bleach dilution too but make sure to rinse this really well. Use a 10% bleach solution, one part bleach to nine parts water and plenty of elbow grease.