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The mosquito plant (Pelargonium citrosum) is not listed as being toxic. I have included a picture to be certain this is the type of plant you're talking about since so many plants are toxic to felines.
While the mosquito plant should not make your pet sick, you should still monitor for any abnormalities such as vomiting, diarrhea, seizures, blood in the urine, etc.
As a sidenote, I find it rather odd that a cat would want to eat a plant usually kept in the house to deter cats from that area. Unfortunately, because cats love grasses, when they're kept away from it--they'll sneak any greenery they can get--even if its toxic. Please visit http://www.citycats.biz/. This is a link to a company that makes purchaseable good types of grass that your indoor cat can eat that won't make him/her sick. Grasses are healthy for cats and help to move food through the digestive tract. It also has a natural ability to "calm the stomach", which is why dogs and cats frequently eat it when not feeling well. In this case, I'll bet your cat just has the munchies.
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Thanks so much!