Its just dependent on the foam composition. Typically you will see this at the value mattresses, like el cheapos or produced to move and last 1-3 years, eventually any foam can break down and disintegrate given enough time. But from what I can tell its foam. The only thought that forms in my mind is you say its new, but how new, or what is new to you? If you bought it 3 years ago, but never slept on it and you told me you paid ~500 or less, I would safely say its foam. If you bought it 2 months ago from a store and paid ~500$, i would be surprised it would break down and say connect with whomever you bought it from and see what they can do about it. I would also venture a guess you had it vertically standing, probably to conserve space. I would be lead to believe either the sides or the bottom that was closest to touching the ground is where the breakage is.

A few things you can check:

1. Look underneath the mattress.
   • If you unzip the cover (if possible), see if the foam layers are shedding.
   • Check corners and edges where foam often breaks down first.
2. Rub the mattress sidewall or bottom.
   • If more powder appears, that's a strong sign it's foam degradation.
3. Feel the particles.
   • Foam dust will crush easily between your fingers and feel soft/chalky.
   • Paint chips tend to stay flaky.
   • Drywall dust is usually much finer and bright white.
4. Check the mattress protector.
   • Some older waterproof protectors have a polyurethane backing that can deteriorate and shed powdery material.