The rims are probably aluminum alloy, but it might have steel eyelets that the spokes nipples sit in. The nipples should be plated brass, and the spokes will be plated steel or stainless steel. You are most likely seeing corrosion of the eyelets. I'd say clean it off with a stiff brush, put a drop of oil on each spoke where it meets the nipple and the rim, and ride the wheels. If spokes don't break and they stay true then keep riding!