Does anyone have any ideas as to what could be going on with this ceiling?
Is that a portion of a rebar that is exposed? If so, that is dangerous. That means it and other rebar is improperly installed. Depending on what is above, the floor/ceiling could be overloaded and come falling down. Concrete is designed to fail by tension (cracking from below) rather than fail by compression (exploding under pressure,) but I wouldn't trust it. If one rebar is installed improperly, (installed too close to bottom of slab,) then I'm sure many other rebars are not installed correctly. Obviously it's been failing for some time (note the rust) but if a new load is added, (snow, waterbed, etc.) it could trigger a disaster. Get a professional to look at it immediately.
Efflorescence. It's described, explained, and discussed, ad nauseam, on the internet. efflorescence-cause