[Airjer_]

take a blow gun and blow all the sand and dirt that has accumolated in the fitting. Push the black part to the right (as it is shown in the picture you posted). You will know it is far enough when it bottoms out on the metal part of the line (you can see the metal line in the grooves of the black plastic piece. Then pull off the line from the filter.

Now that it said they never go that smooth! they usually either break or the line is rusted to the point where it can no longer be removed from the fitting easily. If the black piece breaks it can be removed completely and a quick disconnect tool can be used to get the line off. More often than not I usually break them off, pick out all the pieces, blow out all the dirt, spray liberally with penetrating lube, and use a quick disconnect tool. Then twist, pull, push for the next 20 minutes until the 2 separate.

I hope it goes smoother for you!