Could be bacterial infection, parasitic infection, acidic ph, high temps for a rainbow trout, poor water quality, injury, mycobacterium, and cataracts. Etc, could be lots.
For anyone who comes across this in the future, it ended up being a bacterial infection. Due to the fact that it was unilateral, I think it may have been due to injury. I initially worked with large water changes, daily and even sometimes 2x per day with no effect. Then increased the salt content, slowly over a few days, again with no effect. Finally I used Furan-2 which showed slow progress over a few days and is now almost fully resolved (it got much worse than in pictures above intially).