I generally issue a word of caution to people getting tattoos in languages they don't understand. Besides the chance for typos and other errors, they seldom have the effect of what you wanted to say in the first place. Pink Floyd's lyric works best as a tattoo in English than in Latin, in my opinion. But if you are set on Latin, there are some problems.
One problem is that there are a multitude of ways of expressing this idea. Just like for "lunatic" Pink Floyd could have theoretically used "madman, crazy person, insane person, psycho," etc., there are different synonyms for these words in Latin. And sometimes what might seem a straightforward definition at first glance can be deceiving.
For example, there is a word lunaticus in Latin, meaning "crazy person." However, it is also the word chosen to translate the Greek σεληνιάζεται, meaning "epileptic," in the Vulgate translation of the New Testament. Word choice matters, and unless you're familiar with a wide range of Latin texts, you might not pick up on that.
That then goes into the question of for whom is this tattoo? Are you trying to show it off to a group of people who would know or care about the connotations behind lunaticus, or even if it were correct? Or is it just for you to get a close approximation in a different-sounding language? All that goes into translation.
If you don't really mind the connotations, and want something recognizable, why not go for:
Lunaticus in gramine est. (The lunatic is on the grass.)
Another fairly straightforward way is instead of lunaticus choose insanus, amens, delirus, furiosus, vecors, or excors.
If you really wanted to point to someone in particular who is on the grass, you could add ille before the words: ille insanus in gramine est ("that crazy person is on the grass"). This would be coming from e.g. the wards or doctors talking about an escaped patient from an asylum. If you wanted to make it in general (such that only crazy people would be on the grass), you could instead of ille add homo, which is the Latin word for "person." But then you might get some really crazy looks for having a tattoo that reads homo insanus in gramine est.
These are all problems you'll have to deal with when selecting the best option for you.