Does you cat have any other symptoms: lethargy, vomiting, no interest
in food? If it were me, I'd take a sample of the stool to the vet for
analysis. It could be nothing, but you just never know, and a stool
analysis is inexpensive.

have you give any new food or sweets to the cat

Like the others said, I would take a sample to the vet for analysis.  Get a
sample that is as "fresh" as possible, and place it in a clean plastic bag.
However, I would also look around to see if your cat could have eaten
anything different.  For example, you said the stool is dark green, so look
carefully at any plants to see if they have been chewed -- and that includes
*artificial* plants.  Some cats will also eat them.  And what about ribbon
or thin fabric?  Anything green that your cat could have ingested?

MaryL

Dark green sounds strange but look at this from a google search:

"The liver constantly secretes a bright green fluid called bile
into the small intestine or it is stored in the gallbladder.

Bile is needed to absorb fats and fat soluble vitamins. It also
helps to soften stools and is responsible for giving stools their
characteristic brown color.

As bile makes its way through the intestines, it progressively
changes color from green to yellow to brown, due to the action of
bacteria in the large intestine on the bile salts.

Green stool often indicates that food has passed through the
intestines faster than normal (called decreased bowel transit
time), before it could be changed from green to brown.

Diarrhea decreases bowel transit time, so any condition that causes
diarrhea can result in green stool.

Other causes of green stool include:

Laxative use
Antibiotic use
Medication side effects
Food poisoning
Celiac disease
Ulcerative colitis
Crohn's disease
Malabsorption
Irritable bowel syndrome
Bacterial overgrowth
Infectious diarrhea - especially salmonella and giardia
Traveler's diarrhea
Cancer

Foods and supplements that can cause green stool include:

Chlorophyll
Iron supplements
Algae"

http://altmedicine.about.com/od/gettingdiagnosed/a/green_stools.htm

If the stool was soft and mushy- bordering on diarrhea, the green color was
bilirubinin that wasn't completely metabolized. This happens when ingesta
passes through the intestines faster than normal- like when a cat has
diarrhea.

If it only happened once, I wouldn't worry too much about it- just be sure
to mention it to your vet.

Phil