Iodized salt is just table salt with iodine added. The flowing agent is not a concern. Your horse would get more aluminum from a good mouthful of hay.
Rock salt refers either to the form (small chunks) or to "raw" salt, contaminated with other minerals, the form it is in when taken from the ground. Many different colors are possible. The pink in pink salts usually comes from salt loving bacteria trapped in the salt. That particular bacterium gives it a pink color.
