Is a glow in the dark necklace or bracelet muktzeh? Would any action I needed to take matter if I was already wearing it before Shabbat started?
Per Rabbi Chaim Pinchas Scheinberg it is permitted to use and play with glow in the dark toys on Shabbos.
As far as moving the jewelry, even if glow in the dark would be problematic (which I don't believe it is), it is used for jewelry more often than the glowing (you wear it at night when it doesn't cause glow), and would be considered a permissible instrument (keli shemelachto l'heter) and would be permissible to move for any minimal purpose.
A Shabbos melacha has to fit into some category of Torah or rabbinic prohibition. wearing glow in the dark jewelry doesn't seem to be any more problematic than creating a sound by closing a door. If anyone knows of a category this would fit into, please share.
(BTW, many watches have radioactive materials to make the hands, etc., glow)