Go ahead and grease away. Also, I have a brand new set of calipers for sale
JK, it is a bleeder valve. In fact it is much easier to open your calipers if you crack that valve open then push the piston back in the caliper. Just attatch a hose from the end into a can. You may need to re up your brake fluid afterwards, but it also gets some of the crud out of the system too.