There is a small soft rubber cap/bung/plug on our hot water cylinder that is very slowly leaking water.

Does anyone have any idea what this is?

I can't get a decent photo of it sorry, due to it's location.

It's made from soft black rubber and is about 1 inch in diameter. There is a very slow leak from it. If I press it a few drops dribble out.

Hot water cylinder seems to be working fine.  The surface isn't hot to the the touch.

The label says it's a 2009 Peter Cooks non-ventilated cylinder with an operating pressure of 500kPA.

I'll get a plumber out tomorrow, but should I shut off water to the cylinder in the mean time?