Found a green knob in closet?

In our downstairs closet at the bottom of the wall by the door there is a green knob sticking out of the wall. It look slike a water valve you would turn off or on outside.

We do have a 1/2 bath right behind that wall but if we turn it many times right or left it doesn't seem to make any difference. Anyone have any idea what it might be?

The reason is I am needing to replace the water inlet fittings to our toilet in one of the bathrooms upstairs and I need to turn the water off. There is no valve under the sink upstairs.

Any help would be appreciated.

It may be a water shutoff. If so, it would close clockwise and open counter clockwise.
If you have an expansion tank, it would take a while for the water to quit draining down. Those can hold about 2 gallons.

Thank you for the info. When I get home tonight I will give it a try.

That was it and it worked. Thank you!