If you catch it soon enough after the split, you may be able to just pry it away with your fingers. Otherwise, the aforementioned "anemone cannon" works well. If you are unfamiliar, it's a PVC pipe with a sponge on the end of it, the idea being you place the pipe segment over the nem using the sponge to form a seal and eventually the nem will crawl up into the pipe to get to light.
If you can remove the rock, you can also hang it upside down over a bucket and it will drop eventually. It may take several hours, you can wet the nem and the rock with baster or pipette if necessary.
1)Gently massage the foot where it’s attached starting at the outer edge .
it will take time but eventually it will start to let go .
after the edge is free , slowly and gently work your fingers under it .
2) use a power head pointed directly at its foot. It will become irritated and start to release its hold .
3) ice cubes in a zip lock bag and again slowly and gently massage around its foot .
4) leave it where it is and wait for it to start traveling . When it does it’s hold won’t be as strong
If it retracts to 2", you would need at least a 2" pipe. Trick would be to not have the pipe all of the way down the sponge. Leave a gap from the "base" of the sponge to the rock, so it makes the nem retract and it should be able to crawl up the pipe with no issues. If no pipe that big, you...