Remove the gas tank, pretty sure there is a nut behind there.
It's hard to tell from the pic, but does the centre take an allen key? You may not need to get to the back if that's the case.
The tab pointed to, is locking against the little crossbar just underneath it. It's entirely possible that there is another tab on the other side.
Some of those are designed for you to push in further down, underneath the little crossbar. Doing so causes the tab being pointed to, to move inwards and release itself from the crossbar.
Some of those are designed for you to squeeze the other part of the connector which forces the little crossbars to move outward and away from the tab pointed to. I think that might be the case for the one at hand.
Sorry, I don't have the ability to add graphics to photos; maybe someone who does (and understands what I'm trying to suggest) can show where you should try squeezing.
Its either a drain, or a vent hose.
Trace where it goes.
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