I have had a similar problem before. The elbow has been on so long that the gasket in the end of the elbow has bonded itself to the outlet. Either the oils in the gasket have welded it together or the lack of moister between the two have caused if to stick tight. You will need to use more force to break the bond. However, you risk breaking the tabs on the elbow or outlet.
When I had bayonet fittings stuck like yours I have been able to get them apart but the gasket had been crushed so long it would leak when reconnecting. You used to be able to get the gaskets as replacement parts. However, there are at least 2 different ones used now and are not interchangeable. Another problem is some of the gaskets come glued to the fitting so it can't fall out. The last time I wasn't able to replace the gasket because i couldn't remove it completely and had to replace the fitting.
I would recommend if you can't twist them apart to use a screw driver to break the clear tabs off then you can probably pull the elbow off. Better to ruin the elbow and buy a new one than to break off a round tab from the outlet and having to fix or replace it.