Sounds like a mold/mildew condition from lack of ventilation .
Is anything else inside that garage getting a white film on it ?
Make sure you don't have a leak somewhere that has settled then shows itself while in storage. Check inside all cabinets, use a flashlight and your nose.
You can buy a test kit for mold at H. Depot for $10-15.00
If you have electric, run a dehumidifier.
Buy a 4 pk of DampRid from Amazon which does not require power to operate.
or if you have a boat dealer/marina they will have dehumidifier systems of various types.
Open your roof vents and windows if you can during inside storage.
google ways to clean it off so it does not comes back. Like a vinegar solution.