Rear hydraulic disc sticking
The advice I have seen for a stuck piston is  a) hot soapy water, followed by Isopropyl Alcohol, followed by a small amount of brake fluid (Use the same that is in you brakes - mineral in your case) to lubricate the piston. If you google for "Stuck Bicycle Disc Brake piston" there is quite a lot of useful advise

Warm soapy water on it's own cannot hurt. Although I have never needed to do this, I would use a syringe to direct the water at the pistons to waste away as much crud as possible. Move pistons out (being extremely careful not to drive them out too far - put a pair of old pads in before squeezing the lever... When pushing them back in, do it gently) Watch for a piston moving more than the other. If one is not moving, hold the other in to get it free.  Then squirt water over them and wipe them off with a clean rag. 

If the crud contains too much in the way of oils, the water may not be enough  - resort to the IsoP. 

Wipe a clean rag soaked in a small amount of brake fluid over the pistons. If desperate, a normal mineral oil, but what every you do, don't use too much. You need just enough to put a thin layer over the piston, and more and you risk getting it on the pads (very bad for them). 

Finally, and most importantly, get a clean dry rag and wipe the whole assembly down as clean and shiny as you can. Install pads and see how you got on.