Morris, 26, joined Wildcats in pre season on a one year deal from Kingstone Press Championship side London Broncos.
A club statement said: “Following medical advice of the highest order, Sean has been advised to take a break from the game.”
The statement quoted Morris - who did not play a senior match for Wildcats - saying: “Under medical advice I have decided to take a break from the game for the foreseeable future to make a full recovery from my head injury.
“I would publicly like to thank chairman, Michael Carter, the club and coaches for their support over the past few months with my recovery and going into the future.
“I will always look back to my time here and wish that I had been able to pull on the red, white and blue shirt, but the boys know that they have got my continued support and I wish everyone at the club the best of luck.”
Wildcats chairman Michael Carter said: “I would like to wish Sean all the very best for the future.
“It is very unfortunate that we did not see him pull on the red, white and blue shirt but his long term health and well being is of the utmost importance.
“Common sense dictates that we follow the medical advice.”