The collar is designed to keep any water that may come behind tile to divert around the collar and continue down wall
You can cut the square around the delta valve plaster guard. Or simply discard the square plaster guard and cut a 3″ circle. Either way, I’m pretty sure the square plaster guard has to go to all the collar to sit flat against wedi
These collars are kind of overkill. I’ve never had an issue with water getting behind valve trim without them. Really up to you whether you even want to bother….
They are part of the 10 year warranty for wedi though