They’re not Fel Crystals. They’re Mana Crystals colored green. The same thing was on the Isle of Thunder and the Blood Elf NPCs there mention that they were colored to be more, ‘aesthetically pleasing.’
wow.gamepedia. com/Sunreaver_Magus
Sunreaver Magus says: It is days like this I miss using Fel magic. Sure, it was evil, but it got the job done.
Sunreaver Magus says: Someone told me these crystals are blue in their natural state. I am so glad someone took the time to apply some proper aesthetics.