I don't see why microalgae couldn't be a main food source (putting aside the taste factor).
It contains many different lipids, carbohydrates, proteins and other essential nutrients that are digestible and would be beneficial to humans. I do know that microalgae has a very high lipid content (hence why they're often harvested for biofuels), but it can nonetheless provide nutritional value (ie. feed a starving man).
However, not how this is completely different from living solely off of microalgae. This likely wouldn't be possible, as us humans cannot synthesize all nutrients that we need from scratch. These nutrients are deemed essential nutrients. This problem usually arises when your diet is too restricted to certain types of food. So whether it's microalgae or any other type of consumable, you wouldn't be able to live by only eating that thing.
Another related problem is that growing microalgae is more difficult than it would seem, as is once again usually encountered in second-generation biofuel manufacturing. Temperature, light, cell density, nutrients, and much more must be precisely managed to obtain a increasingly growing population of microalgae. The pros and cons of farming microalgae for billion of hungry people would have to be contrasted with other current agricultures, like corn, soy and rice.