I think Wittaj is correct in thinking it is a layer of the lens cover itself. Were this water spots or errant paint overspray, it would also appear on the black housing. I don't see it on the housing.
I'm afraid you are going to need to do more aggressive cleaning than any of us like performing. I have not cleaned a plastic PTZ lens cover, but have done so on a badly hazed car rear camera plastic lens cover.
The two products I have had very good success polishing up plastics are...
Meguiar's Mirror Glaze Clear Plastic Cleaner - use this first to remove fine scratches
Meguiar’s Mirror Glaze Clear Plastic Polish - for final mirror optical polish
Both can be used with a microfiber cloth. Use a separate cloth for the two levels of product.
I would first try with the gentler, ultra-fine grit polish and see if that is sufficient.
If more aggressive cleaning is needed, use the cleaner and then finish with the polish.
The cleaner will leave the plastic with a haze. It is the polish that removes that haze.
With the older, glass lens windows, I could go as aggressive as cerium oxide, but that would not be a smart idea for the plastic covers.
For stubborn water spots, I have had improvement with several rounds of an acid etch shower door cleaner (3D Eraser Gel Hard Water Spot Remover), but beware that once you have that level of water spots, the glass window itself is likely surface etched, not merely soiled. You may need cerium oxide to polish out.
I've not tried 3D Eraser Gel on the plastic covers, but the plastic should resist the acid adequately.
