Two initial questions. How big is it? How big is it really?
If it's really small just build it up with filler and with the final layer either skim the wet surface with the edge of a credit card to make it level or wait until it is really dry and use a block to sand it down to the original wall.
If you've got crumbly cement it may mean the top layers have "blown" away from the block and are just being held in place at the edges. Penetrating damp can cause this as can other factors. Tap a good wall with your knuckle and it will sound solid and dead; tap a blown wall and it will sound light and hollow. Use the tap test to work out how big it really is.
Next question: is it your own property or are you renting? If it's your own, you owe it to yourself to do the job properly. Find the cause and stop it. Pull away the loose material - there may be quite a lot of it and you may need to replaster but once it's done, it's done. If you are renting, you know your landlord better than us. If you think they will appreciate your help then tell them all about it. If anybody is to blame it's probably a leaky pipe in an apartment above you or the building constructor but if you think they will try to blame you and keep your deposit then follow the suggestion in paragraph two then be very, very careful until you move out. Don't slam any doors!