I accidentally know this word while reading a manga in English. I tried searching it on the Internet but nothing comes out. Yet I found that maybe this word was originally "spectromancer" since there are no websites containing the word spectremancer All of the information about spectremancer is little:
"But there're others too, like that weird spectremancer."
I believe this word describes a type of person or an occupation.
[Source: MangaRock {Chapter 26, Page 8}]