Art print preservation?

I have several signed and personlized limited edition prints that are nicely framed with materials that are supposedly acid free. They’re large and (at least for me) expensive pieces that are important to me. I have two or three pieces that have a light brownish marring (mildew?) under the glass. These haven’t been stored in an attic or garage – they’ve been hanging on my walls. Is there anything I can do to repair the damage (or at least stop it)?