WHY IS THIS SLUG GREEN?

WHY IS THIS SLUG GREEN?

Observation - WHY IS THIS SLUG GREEN? - UK and Ireland. Description: Although this slug does not look green in the picture - it was distinctly green to the eye. Its excrement was green too. (That does show.) It was about one and a half to one and three-quarters inches long. It was curled round when I came across it and

Although this slug does not look green in the picture - it was distinctly green to the eye. Its excrement was green too. (That does show.)

It was about one and a half to one and three-quarters inches long. It was curled round when I came across it and might have measured longer once it had started to move along.

As well as being green, it had irregularly sized cream blotches in rough lines. It was in an upturned flower pot on earth. Is it green because of something it has eaten? If so, why are all slugs not green? Is it the kind of slug? Is there something wrong with it? Have never come across green excrement from a slug before.