What is the most common type of algae?

There are tons of common saltwater algae types, including the same types we see in freshwater. Those include;

Hair Algae, Brown Algae (Diatoms), Green Algae, Red Algae (Cyanobacteria), and what is known as Bubble Algae.

I always see ref algae on saltwater tanks in public or even private areas.

I had no idea algae could be a problem in a saltwater tank. I guess I assumed because of the salt content in the water, nothing like that could grow in it! You learn something new every day!