When one hears the word algae, they tend to think of a green substance you can pick up and hold in your hand. In this case, mustard algae is the complete opposite. Mustard algae looks like dirt or sand that can be found on the bottom, sides, and seams of your pool liner. The defining factor when comparing mustard algae to other types of algae is that you cannot physically grab and or hold it in your hand. If you were to go down to the bottom of your pool and try to touch or pick up, what seems to look like sand or dirt, and it puffs and disappears into the water, you have got yourself mustard algae.