There are many types of non-electric light (aka cold light) that fall under the category of ‘luminescence‘; fluorescence, chemiluminescence, photoluminescence, bioluminescence, and more. These are the most common forms of luminescence you will see.

Fluorescence = road signs
Chemiluminescence = regular glow sticks
Photoluminescence = long-lasting glow and other commercial goods like your LÜMI
Bioluminescence = enzymatically produced light, most commonly found in the deep ocean and if you’re lucky, on a beach near you!

This article is about phosphorescence, a class of photoluminescence where photons are absorbed by a material creating an ‘excited state’, and then re-emitted in a lower energy state. This type of luminescence is used for really awesome things, all reduce our reliance on electricity and are incredibly durable. For example, the same material that makes your LÜMI, is also used in aggregates that can be embedded in pathways. The result was a glow-in-the-dark pathway that was charged with light during the day (solar powered!) and glowed all night long. All of these materials are able to glow due to a very special compound called Strontium Aluminate (aka magic dust, your pick.)
There are many varieties of available, with the highest grade (ie. brightest and longest lasting glow) being of the Strontium Aluminate variety which has been doped with other heavier elements.
It’s definitely not edible (strontium can out-compete calcium if ingested and cause bone fragility), but it is 100% non-radioactive, biologically inert, and impressively benign overall as far as chemical compounds go.