When window frames react with oxygen over an extended period of time they are said to be oxidised. Exposure to the sun and salt air contribute to this process.
An oxidised aluminium frame may not be obvious until water is applied to it. Water that makes contact with the top of an oxidised frame and then runs on to the glass below will leave a very distinct milky stain.
This is no cause for alarm as the window simply requires a more extensive clean than would otherwise be required. Although it is important that both clients and window cleaning professionals are aware of this issue as it can increase the time spent on the job and corresponding cost.
The good news is that a regular cleaning regime with water fed pole technology can greatly mitigate this problem and keep frames and windows looking beautiful.