Yes, this is an interesting little mystery.... The version you've quoted definitely comes from Martial; specifically, from the 1920 Loeb translation by Walter Ker. (
http://tinyurl.com/ygu9ksd) However, a Google search also reveals that the same quote is all over the internet, and usually attributed to Marcus Aurelius (probably 9 times out of 10, though a few people do mention Martial). Without spending some time in the library, I'm not sure how to confirm whether or not Marcus Aurelius also said it.
Anyway, since you wanted it in Latin, Martial is already in Latin, and nicely versified -- so perhaps that solves the (immediate) problem for you:
Quod sis, esse velis, nihilque malis;
Summum nec metuas diem, nec optes.If there is in fact a (different) Marcus Aurelius version in Greek, we could also translate that for you. I would want to see it in Greek first, so we can make it as accurate as possible.
EDIT: PS: In case you're not familiar with Latin verse, this is how the Martial poem scans:
(
- = long syllable, u = short)
- - - u u
- u
- -