Well, you add back water matching your current waters salinity (temp and pH as well). If your current water's salinity is too high due to evaporation then you'll want to add water with slightly lower salinity, in order to attempt to bring the overall salinity of the tanks water to your desired level.
As far as the nem, if it is climbing that high up the glass then chances are your lighting is not good enough. Or your flow is not sufficient. Try changing the direction of your powerhead or add stronger lighting to your tank. When you cause a nem to loose it's footing you not only risk tearing the actual foot (which is lethal 99% of the time) but you risk dropping it onto rocks which can injure it, as well as stress the hell out of the thing. Anemones do not handle stress well. Above anything, if it were me, and I could not get the anemone to move to a better spot on it's own, then I would let him be exposed to air for a few seconds rather than prying him off of the glass.
What is your lighting like right now?