Using just power from ethernet cable for security camera?

I don't believe you need to spend the big bucks for POE, as your requirements are straightforward. If you would install CATx cable for POE, you can easily enough install a pair of wires and add a micro usb pigtail at the camera end. Even though you are running only 5V, to reduce voltage drop, consider to run something as large as 16 gauge wire to the pigtail. Ensure that you provide correct polarity on the added wiring, of course.

this is how PoE is usually used (picture below)

the PoE injector feeds around 50VDC into the cable

in your case you could also use the ethernet cable to carry power

connect a 12V power supply at one end

use an automobile USB charger adapter at the other end to power the camera

I've done this before with Dlink cameras. Cat 5 cable will have 4 twisted pairs. Two are used for data and the other two for other things. I used one pair for positive and one pair for neg. I simply cut the power cable and spliced in the source somewhere near the router and spliced in the termination end right at the camera. So all that ran to the camera was the cat 5 cable. I used zoneminder for 4 camera this way. Ghetto POE, but it works.

I tink a PoE splitter is your best bet.

I have a wifi camera that's connected to my network using wifi, but gets power from a POE splitter. You'd need access to either either a PoE switch or injector, properly wired Cat6 cable, and the PoE splitter.

The POE splitter's input jack takes a Cat6 cable that comes from my PoE switch in the attic, down to my front porch, and the splitter splits the PoE cable's power+data into a micro USB power plug (power), and Ethernet data via RJ-45 connector that is left unused (notice it's taped up).

note: However, I will say that the setup as pictured is not ideal. You want to not expose the splitter or the cables. In the future I plan to change this camera with an actual PoE camera, and hide the cables better.