While white specks on the glass of a saltwater aquarium may be unsightly, and sometimes difficult to remove, they are usually signs of a healthy aquarium. While you will likely want to remove them from the front and possibly sides of the aquarium for viewing purposes, don't clean the back panel of the aquarium to allow these organisms to thrive.
If the specks look similar to white sand and move on the glass, they are likely copepods. There are about 12,000 different types of copepods, a type of crustacean, but the most common ones found on glass are the harpacticoids. They only thrive in healthy aquariums and seeing them on the glass is a good sign you are taking proper care of the aquarium environment. Copepods will help remove some types of algae. Additionally, several different types of fish and invertebrates will eat copepods, some exclusively (like the mandrinfish), and copepods are very nutritious for the animals and some aquarists deliberately breed and add them to their tanks.
Amphipods are similar to copepods but they are bigger, about the size of a large comma. Amphipods can also be a great source of food for aquarium inhabitants, but some amphipods are parasitic and will harm fish. Amphipods help clean up waste in the aquarium; however, if they start to become a nuisance, it is an indication you are over-feeding the fish.
Coralline algae is a type of marine algae commonly found on live rock. It is usually pink in color, but can bleach white occasionally and leave white spots on the glass and rock. This type of algae uses calcium in its cells and can be difficult to remove from glass. However, it is generally considered beneficial in a reef aquarium. As the algae covers the rocks, other types of nuisance algae cannot grow and the coralline algae adds color to the aquarium. Good coralline growth is generally considered a sign of a healthy aquarium with good calcium supplementation.
Another common animal that can make white dots on the glass of a marine aquarium are small calcareous worms, meaning they use calcium to build their shells. These worms are tiny filter-feeders that are harmless to the aquarium. These worms can be scrapped off of the glass if you find them problematic. They are easily identifiable with a magnifying glass -- the shells are formed in a spiral.
If you have certain snails in your aquarium (such as Nassarius snails), either purposefully purchased or introduced from live rock, their eggs may appear as white spots on the glass, rocks, and back of the aquarium. In a freshwater aquarium, snails can be pests; however, in marine environments they are usually not a problem as many things will eat the snail hatchlings and prevent them from overpopulating the aquarium. This is particularly true if you have filter-feeding animals like fan worms.
Hydroids look similar to upside down jellyfish and adhere to rocks and glass. You want to remove hydroids from the aquarium, as they can rapidly take over under the right conditions. Hydroids can cause a nasty sting to both humans and corals and upset the balance in the aquarium. There are a number of ways to remove them including boiling water and adding predators.