IMO,
The polyp should be dry, dab it softly with tissue or soft rag. same as the plug your going to use.
Have 2 small cups with your current tank water. Your using this as clean off and curing station.
Put a decent amount of glue on the frag plug, gently place the polyps that your fragging on it. make sure only the bottom of the polyp gets glued.
Then place the polyp that's been attached with glue into the first cup. This cup allows the glue to settle a little and hold on the the polyp. This also assures that any chemicals leaching out of the glue are only in the cup. after a couple mins, place that frag in the 2nd cup where you cure it for another couple mins. This allows the frag to further cure and less chemicals from the glue are left. once complete, place the fragged plug in an area in your tank that has some good flow but not to much that it'll blow the polyp/s off the plug.
If you look at the cups you see film on the top of the water. that's just extra crap from the glue. It's not harmfull to the tank , just less stuff for your skimmer to take out. the cup tecnique just buys your polyp some curing time for the glue to harden a little and hold the polyp better. IN case the polyp slimes off and pops off the glue, you still have it and won't flow away since its in a cup not in your main/frag tank.