There is standard for PoE: IEEE 802.3af and relatives
. However, quite a few vendors came up with their own PoE implementations. In Mikrotik world, "passive PoE" refers to MT's own proprietary implementation, which incidentally is compatible with Ubiquiti's. So in theory RB4011 can power UBNT AP. However, one has to always check power budgets. You're writing that AP consumes up to 17W. RB4011 has PoE out current limit at 600mA when supply voltage is less than 30V and 450mA when supply voltage is above 30V. With originally supplied power adapter (24V) this means around 14.4W output power which is less than AP can require.
In short: it might work, but if you notice any instability in AP's performance it might be due to insufficient power output of RB4011.