Air carrier Flight Attendant reported a passenger was using a portable HEPA filter device in flight. Flight Attendant was unsure if such devices were allowed and requested company provide clarification.

Narrative

During the flight; the inflight service is main cabin was completed; FA3 (Flight Attendant) approached me about concerns she had about a device that a passenger was wearing that she saw during their beverage cart service. The passenger was appearing to wear some type of mask with tubing attached down to what appeared to be some type of box. This was far more elaborate than a surgical or cloth mask; but also did not appear to be a portal personal oxygen container (PPOC); nor did it appear on our final paperwork. FA3 felt that this mask was a prohibited item and asked me to check out the passenger and the mask; and then tell [the] Passenger to stop using the device (I do not know whether FA2 or FA4 also had concerns about this). FA3 then directed me to seat XXC; where the passenger was sitting. After finishing delivering a few drinks to my passengers in first class and reviewing the manual on PPOCs and allowed and prohibited items; I went to row XX to find the passenger and check out the mask. As I approached that row; the seat was empty and the passenger had left the seat and gone the aft lavatories. I waited outside the lavs until they came out; saw the mask; was unsure exactly what it was myself and asked out of curiosity to tell me about what they were wearing/using. Passenger told me it was a portable HEPA (high-efficiency particulate absorbing) filter that they used; and the device appeared to run on battery power. Since the closest reference I found in prohibited items was a personal humidifier (which this was not); I did not confront [the] Passenger about it but watched the passenger go instead and take a seat at XYC then. Without knowing enough about the device; whether or not it was allowed or prohibited; nor where the passenger was actually seated; or any mention of it on my paperwork; I did not take any further actions. I do not recall seeing the device during the boarding process; though I could easily have been looking away; preparing a pre-departure beverage then while they boarded or the passenger was not wearing the device then. I am not sure whether I committed a FAR or Safety violation here; which is why I am reporting this. If the device is allowed on the flight; then flight attendants also need to know before potentially harassing someone unnecessarily for another form of approved mask or facial covering onboard.Suggestions: Can you please provide clarity on portable HEPA devices; whether in a bulletin or a manual? What are the potential danger(s) of these devices; if any; and under what conditions and/or restrictions can passengers use them in flight? Do TSA; agents; and flight crew need to know about and flag these devices for further inspection and or notes on our final paperwork? On the passenger side of things; what can airlines do to make it clearer about HEPA filtration systems already used on passenger aircraft; and make better informed decisions about bring portable HEPA devices onboard?