Sea salt made from current seawater would include microplastics from that water, but rock salt would not as it was formed before these plastics existed. On the other hand, various mines of rock salt may have all kinds of other mineral additions that may be harmful for human consumption, so most rock salt is used for e.g. deicing roads.

In a lot of regions, "farming" sea salt basically means pouring sea water on top of plastic tarps and evaporating water. Over time, the plastic degrades and you get microplastic particles in your salt. You also need to rake these salt fields pretty heavily, which also generates microplastic particles.

Which is sad because salt "farming" used to be done on clay soil near river deltas, which was a perfectly good medium for this job.

i am thinking, most of the salt consumed comes from sea salt that has those microplastics. if we use rocksalt, well that microplastics wont come from seasalt.