This is a common problem with turbos that are attached to pieces of equipment that are stored outside, like marine turbos and those attached to outdoor generators.
The problem is that prolonged exposure to the elements and water causes a build up of corrosive materials (and rust). Apart from causing significant discolouration, corrosion also causes the compressor wheel to become warped, damaged and brittle.
Damage like this affects the shape and surface of the wheel, preventing it from turning easily and reducing both the effectiveness and efficiency of the turbo.
Whilst it is possible to clean small amounts of corrosive material from compressor wheels, the safest and most cost-effective solution in cases like this is to fit a new one.