This, if no peeling of old paint or plaster or patching of cracks.

Mold, some minor leak. What is above the ceiling ?

If you are diy, then paint few coats. My place yellowish leak mark after few coats, looks ok. If want perfect white, then add more coats. One big tub maxilite not expensive. Keep bal also have to throw later. Mix some water b4 painting but not too much. As long the painting swipe is smooth. Too thick, after dry not so nice
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looks like mold. pls ensure the room is well ventilated; and toilet does not vent into the ceiling space because all the moisture will be trapped like tupperware..

if u dun believe.. u wet a towel completely and see how long it takes to dry indoors.. that is the problem with moist air