Leaking saddle valve
Turn the water off, and remove it. Those things are rubbish, and always leak eventually.

If you need a valve there, cut out the damaged section of pipe, and install a proper tee and valve. If you don't need it, cut out the damaged section of pipe and install a coupling. 

If you're not an expert solderer, They make push-fit fittings (SharkBite® is the well known brand) that are easy to work with. These fittings are more expensive than typical fittings, but ease of use and not having to buy soldering supplies might make them comparable.