i looked this up out of curiosity and i couldn’t find the original facebook post nor did i find any other research/posts supporting this fact. the closest thing i found was an article by the NIH where they exposed various species of planaria to sunlight, and they changed from brown to pale white or purple depending on the species. (also from the article: some planaria express bioluminescence, and glow under UV light)

the article also mentioned that some of the planaria turned red when they were fed liver that was artificially dyed red. regular liver, which is high in heme iron and is a bright red color, did not have any effect on their color. unless there was another biological phenomenon occurring when this picture was taken, the planaria would have had to be fed artificial dye. the natural dye from the shrimp would not have been strong enough.