Just thought I would jump in here for clarification. It is a common misconception that algae eating species fit into the classic definition of an herbivore as many people typically define it (vegetable, leaf, grain and fruit eating vertebrates and invertebrates)
Micro algae's are typically quite high in protein. Blue/Green Spirulina algae for example is 65% protein! Green algae/Chlorella 45% protein.... Algae specialists of all kind are used to high protein diets. The problems typically seen when feeding high protein diets to these species is more due to the origin of the protein than the levels. High levels of animal protein are not processed the same in the gut as algae protein, and obligate algae eaters can indeed have troubles with this type of food. The cell walls of many algae species are difficult to break down and likely require a specialized gut to process efficiently.
IMHO, an obligate algae eating species that eats a variety of algae in nature is probably eating a diet that is higher than 35% protein. This is more than twice the level of what you consider a classic "herbivore" that eats normal plant fodder such as leafs and vegetables.