You need a larger group of Emperor Tetra, that is the most likely cause. This fish, like all characins, is a shoaling fish that lives in large groups. I would get another 4 or 5 to make a total of 7-8. However, no mention is made of tank size...this fish needs a group but it also needs space as it is an active swimmer. At least a 30-inch tank. Then up the numbers. If you don't, this behaviour of the male, which is perfectly natural, will stress the females and may kill them eventually. Male Emperors drive females hard, I know, I have a group of 12 in my 90g that I raised from fry.
The light really has no direct relation to this, it is normal characin behaviour for this and many other species. But having said that, bright light is not appreciated by this or most forest fish, so a cover of floating plants is advisable.