A few things going on in regards to light transmission and suspension of salt.
First all these salts are the same as in NaCl. Where they differ is purity/additives. Salt by it very nature is problematic in that it picks up water. NaCl is used to calibrate some instruments that measure water uptake based on its very well studied water adsorption. So unless you store your salt properly and dry it you are adding some water to your plastic during the heating process. Adding some cloudiness to the end product if not all removed (not a big issue as often gets boiled off during heating). Additionally to counteract salts water loving tendencies manufactures place anti caking agents in it to avoid it turning into a brick (and iodized typically). So you have impurities playing a role in regards light transmission. Other issues that cause cloudiness are result of tackling the suspension issue..
The salt crystal shape plays a role in suspension. Table salt and others are cubodial. The shape results in crystals that don't suspend readily. So guys grind it to make the particles smaller but in doing so exponentially increase the surface area and further cause issues with transparency (lack of). Sort of like fill a glass with ice and pour a margarita mix over it versus putting that same ratio in your blender. You also are adding defects in the crystal in the process. Think of safety glass: pre and after hitting it with a hammer.
Kosher, Maldon, and other salts prized by chefs are different in shape. Kosher typically is forced into a flat shape under pressure to form flakes. So take two cubes the same size/mass. Take the second cube and compress it flat. Drop them in a liquid guess which one hits the bottom first. The shapes vary in regards to displacement. The flat shape will displace more plastic and will sink slower than the cube and is the reason Kosher salts suspend better in comparison to table (cubodial) salt. Cargill uses a process called Alberger process to make some of their salts. It results in concaved plate/flake. These salts Kosher, maldon, the Cargill select products are typically larger particle size to boot so often get the best of both worlds... larger crystals (less defects and less surface area) with a shape but do to the shape suspend better than cubes. Additionally they often don't have anticaking agents.