Forester 2009 Glowplugs

I recently had an issue with warning lights, which led to a faulty glow plug being diagnosed. The glow plug won't come out and the garage (non Subaru) were cautious not to break it off! The glow plug has been disconnected  and warning lights now reset. The other glow plugs also appear to be seized. Is there a straight forward solution or do I just live with it? Will the one glow plug that is not working cause damage? I am not an expect but it sounds like a cylinder is noisy until warm. I would appreciate any thoughts. Thankyou