TRP Hy Rd brake : why won’t actuator arm return properly?
I suggest to clean the whole brake and replace the oil. After some time, it accumulates particles caused by abrasion. The oil of my brakes came out completely black.

First of all, you'll need a bleeding kit for this.

Remove the brake from your bike and remove the pads. Open the top lid by removing both Torx T10 screws. Pour out the used oil and rinse the brake with alcohol, e.g. methylated spirits. Clean the reservoir with a rag. 

Now, check if the piston returns the whole stroke by itself.

If it does:
Close the lid and tighten the screws. Fill the brake with new mineral oil by help of a bleeding kit.

If it doesn't:
Open the body of the brake by removing the two Torx T20 screws on the spoke side of the brake and see what's wrong.