When an E-4 calls me "hey" or "hey man" knowing that I am an officer, should I bother to correct him?

Definitely correct him. When I was a wee private, clueless and lost I saluted my LT in the PX; he corrected me and I appreciated that he didn't let me continue to look like a fool saluting indoors. I remember when I was a cadet it was my first day at my new unit and I said, "Hey. Do you know where...." and the LT that I said it to was livid. She assumed that I was saying "Hey you", but in reality I was saying "Hey" as in hello or hi. I explained myself and she explained why she flipped out and corrected me and I appreciated her for it. Correct him and let him know that he is not your peer inside or outside of work. Don't let him get comfortable with speaking to you that way.

This is difficult to answer. I will say if it is at work, you address the officer with "Sir". If you are not acquainted with the officer other than occasional meetings outside the workplace, it is still "Sir". If you are friends outside the workplace, I see no issue with addressing the officer by name or "hey man". I had a unique assignment where everyone inside the workplace called each other by name rather than addressing rank. We worked in a highly sensitive area. Outside of the area, we addressed each other by our rank. My immediate supervisor and commander both called me Bud (nickname). We are all retired now but the friendship remains. Your decision 1LT Jeff R. but if it was me, I would nicely tell the SPC it is proper to address you by your rank and leave it at that.

If it happens again, stick out your hand and say "Hey SPC, have you met LT (insert name here)? He is a commissioned officer and should be addressed as such."

Correct him and I guest in my day 1979 1982 at Ft Riley, Ks. BN level school of the soldier afternoon and week end duty, insubornation required Article 15 and 15 days CCF correction custody facility! But that was oldschool. New school soldier, well correction on the spot and leave along until he do it again!

Regardless where you see each other courtesy and respect must always be upfront..

Never ignorant the failure when you have the authority to correct it and bring it

find an NCO to do what should be done, since you lack of leadership is clearly showing - WTF happen to the Military that we even have situations like this -

Shouldn't let it pass. However, the approach you take could mark you as a leader or just an LT pulling rank.

Honestly Sir, I think if the discussion is just a friendly discussion it might be fine. However, the line should still be addressed with their NCO. If the discussion is about work or anything work related, nope the soldier should address you as sir/ma'am. I also must say I may have said "man" when having friendly discussions with my commander. But I also am very aware of when we are just conversating to pass the time versus discussing business. In addition him telling me their is a time and place, and I tend to do well in regards to both of those. As I would never respond with "yea man so..." in room full of other soldiers either. I think it depends on the setting, also I think that level of comfortability can exists while still having respect for customs and courtesies. In doing so it can also create a healthier work environment if discipline and comfortability can be achieved. I also believe this may help soldiers be more forthcoming with issues if they feel more relaxed.

No. Ignore it. Everyone makes mistakes. But if you detect some serious sarcasm or outright disrespect (which is not what I'm reading here), find out who it is (you mentioned seeing this person before), engage his company/battery/troop/squandron commander directly. Things like this don't make themselves up; the CO will know something's up and engage accordingly.
Other than that, just chalk it up to an enlisted member who doesn't understand basic military discipline and respect due their superiors.

In a heartbeat you correct him, both for protocol and for his future.