All metals will ultimately degrade and return to their mineral state. However stainless steel is very slow to do this because it is much stronger than many other metals. Just like bronze strainers turn green as they begin to deteriorate.
Unfortunately the name stainless steel is a little misleading because it does stain. However this staining is not necessarily signs of rust. Often the metal under the staining is in perfect condition.
Staining can occur on strainers where salt water dries. It can easily be cleaned off the metal with a number of cleaning products just as you would do for polished deck fittings.