Sometimes, when making homemade mayonnaise it can “break” and separate. This means the oil has separated from the other ingredients into two distinct layers. To fix it, add a fresh yolk to the broken mayo to re-emulsify the mixture.
To reduce the chances of having your mayonnaise break, make sure all your ingredients are at room temperature and add oil in a very slow stream while processing or blending after the other ingredients are well-combined.