Answer:

This can be one of two things. It may be a type of algae, probably an algae referred to as mustard algae. This can be treated easily with chlorine and algaecide. It may require some vaccuuming and brushing to get rid of completely, which you will want to do or it will come back. The other thing it could be is metal. Iron can appear a brownish color when it comes out of solution in the pool. It can be treated with metal removal agents and some may also need to be vaccumed to be removed.