I reckon superglue Gel or cotton for ferns and things that will eventually attach. You can be creative with the placement with a blob of superglue.
For all the mosses / liverworts that don't really attach (assume all but Java and Fissidens) use invisible thread from wilkinson or fishing line.
I'm basing this on recent experience using superglue with peacock moss, looks ok..ish there is a white streaking visible to begin with, but the real problem is when the time comes to siphon / waft the moss free of gunk eventually the bond between moss n glue will break. With line you can bind more moss to the slate and create a thicker, more maintainable carpet. The invisible thread from wilkinson really is difficult to see in the aquarium so you won't lose out aesthetically.