I know, usually hard radiation and merpeople don't mix. That's why this is great! Bear with me...
Cherenkov radiation is produced by fast moving particles (usually electrons) in water. This phenomenon produces characteristic far blue / UV radiation which looks awesome. These merpeople are already in the water; check. And one would expect their eyesight to do best with far blue and UV frequencies which are the ones that penetrate water to greatest depth!
But where to get sources for Cherenkov radiation? They are not just lying around... or are they? Fortunately this is fantastic merpeople fiction!
Deep under the ocean, radon generated by natural radioactive decay accumulates. Sometimes these accumulations are trapped in metastable clathrate crystals which can survive being brought topside. The radioactive radon will produce blue Cherenkov radiation suitable for illuminating the cave for some time, gradually becoming dimmer as the radon is exhausted. The cave will be illuminated with indigo, violet and UV frequencies by eerie glowing crystals.
How to safely handle the glowing crystals? Probably with shielding, or tongs, or both. Or short exposure times. Old merpeople who specialize in finding and retrieving these crystals might have growths or cancers on their hands from a lifetime of exposure.
I like the idea that these crystals from deep ocean trenches are produced by life forms, which concentrate uranium salts via biology and live via radiobiology on the emissions produced. The crystals and their contents are made by life forms. The Cherenkov emissions are a byproduct. The merpeople know the crystals are alive, in the way that plants are alive, and treat them respectfully. Once the blue glow fades they bring the crystals back to the deep places where they were found, to regenerate themselves.