I would send it back. Even if a little extra liquid was spilled while packing it, that should've evaporated by now (the time between factory packing and you receiving/opening it should be a few days/weeks at a minimum).
Any sign of liquid is a bad sign with liquid cooling.
Sure, you could try using it, but you may end up damaging your system. AIO warranties are for the cooler itself, not for any resulting damage.
Putting a ~$100 AIO (for example) in a ~$2,000 rig (again, for example) is always a bad idea IMO. Your ~$100 cooler is covered, but you're on your own for the ~$2,000 in other components. No warranty for liquid damage on those components, and insurance is unlikely to cover it either.
If you're dead set on using an AIO, that's fine (I have in the past) - but give yourself the best chance at success by not using one with potential signs of leakage.