It's likely that colour is being used by a placed file, usually a file from Adobe Illustrator.
Check your links panel and see if you have any images from Illustrator.
If you do - go into Illustrator and alter the colour there to a Process colour, not a spot colour.
And when you save and go back to InDesign you should make sure all your links are up to date and then you can remove the colour.
Let me know if that helps.