I have a Hinoki cypress that's been in the ground for about 25 years. I've noticed that some of the newer growth more resembles an arborvitae - straighter growth vs the tighter curled growth of a Hinoki cypress. Some of this mixed foliage is coming out of the same branch. Not quite sure what's going on. If anyone knows, TIA.

You are welcome to provide images...

It sounds like a simple case of reversion, where the selection/cultivar with non-standard foliage will produce foliage that has reverted to the species characteristics.

A very common example of this: Picea glauca 'Conica' (Dwarf or Compact Alberta Spruce) is a very dense short needled selection of this species. Just about any plant that has been in the ground established for 5-10 years will throw a reversion stem. It is very obvious when you see it, and it needs to be pruned out because it rapidly wants to grow into the full sized species tree.

John - thanks for that info. The "reversion" stem is growing faster than the rest of the shrub. Would a stress event have caused this or is it just some random event? Can I prune it out at this time of year or should I wait until colder weather or even spring?

I think you can prune it out any time you like. You might plan to use it for holiday decoration.

John - exactly what I decided! Talk about fresh holiday greens. I didn't want to prune and stimulate new growth going into winter. Thanks!