Pinch the grey squares with your fingers or with pliers. At the same time pull the hose off.

 

NS Thank you. I tried the same way. but It doesn't move. Will there be a problem if I pull it by forcefully? Or should I press the gray button with a bigger force than I thought?

 

I am not a mechanic however the heat passing through the hose often times creates a bond with what it is attached to. A lot of twisting and pulling might free it enough to slowly pull away.
i am from the days when you climbed in and hit it with a hammer. Not sure about doing that today.
good luck

 

Did you get the new part yet? If so look at that. Twisting may not help much at getting it to release because it’s an o-ring inside of it that is separate and that is the part that heat cooked and is causing it to be stuck now. A lot of force may be needed, do whatever you can to not damage the reservoir but if the part is bad and going to be replaced cutting it apart with a side cutter might be the solution or rip the grey locking mechanism out.

Good Luck

 

I'll replace the parts. Thank you. I'll try to cut. Have a good night.