Hi!
If there is a bluish film on the eye surface, then seeing the vet as promptly as you can is important because you are dealing with an ulcerating (i.e. infected) eye injury. They can deteriorate very quickly but also heal the easier the sooner treatment can kick in.
If the cloudiness is inside the eye, then you are more likely dealing with a cataract, which is not uncommon in the elderly. Sadly, unlike humans, there are no cataract operations for guinea pigs to remove the increasingly opaque lens.
Cloudiness in the eye can in some cases also be a remnant of an eye infection that has penetrated deeper into the chamber in front of the eye ball while the surface has already healed up. It is much less likely as you would have noticed signs of an eye infection/injury before as a caring and observant owner.
It is however always better to have a cataract confirmed by a vet. Thankfully, sight is the weakest sense in guinea pigs whereas it is the strongest in humans, so they cope much better with it and learn to compensate with their other stronger senses to lead a perfectly normal life. Make sure that you do not keep their furniture too well cleaned and use surfaces that retain scent well as that will help with the everyday navigation for piggies with cataracts in both eyes.
I've had fully blind 8 year old cataract piggies still enjoying to create a new zoomies loop by laying a new scent spoor and following it happily round and round, even though by then the run was very much an arthritic waddle - but there was nothing wrong with the youthful enjoyment and the zest for life!
If you apply some homemade saline solution as a disinfectant