Low-level light therapy has been studied for years, and many studies prove its safety. Furthermore, many scientists believe that LLLT may be beneficial for multiple health purposes.
The face masks that emit LED lights are considered safe to use in general as they’re not invasive and don’t contaminate the skin with toxins.
However, like with every other beauty device, it's best to purchase a face mask from a verified place. Also, you should immediately consult a doctor if something strange or wrong happens to your skin.