It's probably a filter. If there is printing or embossed lettering around the outside of the rim near the front, that would indicate a filter is screwed on.
Remove the filter by unscrewing it. If it doesn't seem to want to come off, see: How do I get a stuck screw filter off of my lens?
After comments, the filter is a Rocketfish RF-CP67 Circular-Polarising 67mm diameter filter which unscrews using the knurled section highlighted. The section forward of that with the RocketFish logo will turn infinitely - which is how it varies the polarising layer.
