In a well balanced macroalgae aquarium, just like in a well planted freshwater aquarium, the growth of the decorative macroalgaes themselves will normally do a good job of out competing unwanted micro/macroalgae from the tank.
However this is not always the case (especially in new setups) and we have to add some cleanup crew to keep the pest algae under control. This is important to do because not only will the pest algae use up nutrients that otherwise would go to the ornamental macroalgae, but also we can have an issue of the pest algae becoming epiphytic, basically growing all over our nice macroalgae and smothering it.
Just like any other saltwater tank, in a macroalgae based aquarium you will need to add some critters to help keep pest algae under control. The difficulty with this is the fact that the ornamental macroalgae can be as appealing to herbivores as the unsightly nuisance algae. There is quite a lot of overlap in terms of what you can add to a macroalgae aquarium compared to a mixed reef tank, however there are some species to avoid putting into a macroalgae tank which you would put into a reef tank, and the opposite is also true for some others. In most cases you will use a combination of different invertebrates as cleanup crew because most herbivorous fish will east pest AND decorative algae, which is not ideal. There are, however, some exceptions to this listed below.....