Hello everyone.

I am powering a hEX S through PoE with a RBGPOE Gigabit adapter.
I'd like to understand how to use the PoE Out on Port5 to power an Ubiquiti Unifi-IW-HD which accepts 802.3 af/at.

Do I need a special adapter to convert the Mikrotik Passive 57V into af/at ?
Will I need to use a crossover cable to convert from MTik Passive 57V into Unifi's 802.3af/at Mode A ?

If Ubiquiti Unifi-IW-HD is 802.3af/at compliant it should accept both Mode A and Mode B.
And despite the fact, that in it's user manual it is stated that "using of passive PoE injectors is not recommended", I guess that means that it is still an option.
So there is a good chance that it will work from hEX S, if 48-57V power adapter is used.

If Ubiquiti Unifi-IW-HD is 802.3af/at compliant it should accept both Mode A and Mode B.
And despite the fact, that in it's user manual it is stated that "using of passive PoE injectors is not recommended", I guess that means that it is still an option.
So there is a good chance that it will work from hEX S, if 48-57V power adapter is used.

I didn't notice the poe injectors not recomended... That's good news.

Anyway, I don't believe there is anything like Mtik 57V passive to 802.3 af/at ?

There are converters from 802.3af/at to lower voltage passive PoE (Mikrotik has one too).
And there are converters from 24V Passive to 802.3af, but they are quite pricey and I don't know if it is worth it, or it's just easier to by a PoE switch.