What is mustard algae will it cause brown staining on the sides and bottom of an above ground pool?

The brown staining most likely is from iron in the water. You must be on a well system or the city water is coming from different wells. There is a product that will remove some of the staining depending upon how old it is. At the moment can not recall the trade name of product.

Mustard algae is treated with high concentrations of liquid 12.5% chlorine and a whole lot of brushing. With a vinyl liner you have to proceed with caution on the strength of each product that you use for the remedy. Best to consult a local pool store for the exact road to take. k