Mildew, musty odor from vent. How to fix it?

Just had the problem this week as the weather is warm and used the A/C first time on Sunday. Now when I turn on the fan, there is a strong odor coming from the vent with or without the A/C on. I am wondering what's the cause?

Should I take the car to the dealer? I bought this used volvo xc70 four months ago and it's still under warranty. Should it be covered under warranty? Thanks!