To further expand on the matter, splitters that take the 24V in and output the 12V for the camera do exist, BUT there is even another issue, Mikrotik (passive) PoE is “mode B”, it has to be checked if the splitter is mode A or mode B:
https://www.etherwan.com/support/faq/what-poe-power-pins-assignment-mode-b
(and it is surprisingly difficult to find the actual specs of these devices)
Sometimes these modes are called instead “mid-span” and “end-span”.
And some are 10/100 whilst other are 10/100/1000, usually the mode B ones are 10/100, while the 10/100/1000 are both modes A and B but are intended for 802.3af (48-56V) and not for the passive 24V.
Example of a “declared” mode B 10/100:
https://shop.poetexas.com/products/poe-12v10w
Or one could take a Mikrotik RbgPoe which can work both as an injector and as a splitter:
https://mikrotik.com/product/RBGPOE
http://forum.mikrotik.com/t/mikrotik-rbgpoe-usable-as-poe-splitter/152794/1
and use an el-cheapo 24V to 12V step down converter (the kind used for trucks are rather handy) and a couple jack connectors.
On the other hand, a test if there is PoE out when it is powered at 12V (probably one would need some 14-15V to actually still have 12V at the other end of the ethernet cable, it depends on distance and quality of cable) would cost very little or nothing.
Yes, there are two holes on the back for wall mounting it.