To clean a salt water aquarium, very similar to a fresh water aquarium, I recommend doing it very, very regularly. You don't want to let a salt water aquarium's parameters get out of whack. You don't want the pH to drift out of range, your nitrates to get too high. You don't want to shock the system.
The idea is to maintain a good environment. The best way to do that is with weekly maintenance. Now, that doesn't mean you have to devote an hour or two every week to the aquarium. It does mean that you have to devote 20 to 30 minutes. But, if you love your aquarium it should be no problem. You should enjoy doing it.
So, to clean your aquarium you want to clean the algae, obviously. You want to always have the viewing panel look as good and clean as possible. To do that just use a regular algae brush. Just be careful if you have an acrylic tank that you don't use one intended for glass as you will scratch your acrylic. A lot of the cleaning magnets work really well. One goes on the inside, one goes on the outside, and you can run the magnet along the outside of the aquarium without getting your hands dirty. That cleans the algae off the inside viewing panel.
Also, when you do regular water changes I recommend gravel or sand siphoning to get a lot of the waste, detritus, and fecal matter that's been building up in the sand bed. You want to free that. Siphon it into a bucket. You could also pull out some of the decorations before you do your water change as a lot of the waste accumulates under the decorations. Then the decorations themselves, depending on what they are and how they look, some of them can be cleaned too.
But you don't want to go overboard. If you're going to do a water change and clean the decorations I recommend not cleaning your filtration until like a week later. So it's good to do it on a schedule where you do your filtration one week, the water change on the following week, because the goal is to minimize the shock to the system.
Every time you do something to the aquarium you're shocking the balance of the system. The bacteria that's been growing in there doesn't like to be shocked, and if you shock it too much it'll result in some die off and you could have an ammonia or nitrite spike a couple of days after you do some aquarium servicing.
So, keep all those tips in mind and, again, minimize the stress to the aquarium.