If the fork is new, it could be normal. The seals are often coated with a grease that will rub off on the stanchions as you ride. If it's got some miles on it, it may be leaking oil and be in need of new seals.
If it is a lot of oil/grease you should probably be concerned but if it is just a light coating its probably nothing to worry about. If in doubt, get your LBS to check it out sometime. They would probably look at it for free if they are a decent shop.
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