As a plastics engineer, I would be more concerned with the heavy metals in Himalayan pink salt. Also, any microplastics wouldn’t be “absorbed” If anything the salt would abraid the walls through shaking the container which could scrape the walls and grind our some small particles over time. That bwing said, the plastics used for these types of applications are relatively virgin and frequently don’t contain any additives aside from possible colorants or glass fill or something line that.