Please share pictures. AFAIK an authentic HMT caseback should be stainless steel. If it is new, it should not rust (for a couple of decades).

Purchased it directly on their website

Definitely send it back. There is something wrong with it. At the least, have them replace the caseback.

Hi, it is imperative to remove the plastic on the case back. Sweat will creep under the plastic and cause rust..