How to remove chemical stain from granite composite

The white stains might be dried on soap scum, i.e. calcium etc. salts of detergents. In that case, a vinegar or citric acid solution should solubilize them and allow you to rinse them off with a hose.

A product called CLR (Calcium, Lime & Rust remover) is really good at getting rust stains off tile, granite, concrete and toilets. It is available at most grocery stores in my area. make sure to rinse well after the stain vanishes (it works very quickly).