don't let water dry on the lens.
Just kidding...
Cleaning lenses is a delicate job, with the major problem being the risk of scratching. Scratching happens when you use something harder than the glass or the optical coating. I don't know what coating this lens has so I can't advise you on that part, but most lenses built for UW use are hard enough to sustain some solution cleaning without danger.
I would start with just a little warm water. Soak it for a bit, too, just to soften the calcites or other deposits that are forming it can reabsorb the water they were once dissolved in, and then gently pass a CLEAN tissue or very soft cloth over it. I'm a bit of a risk taker, but if it's really bad I might fill the sink, put the whole lens under water, and gently rub the tip of my finger over the spot and see if that will loosen it.
Another easy and viable option is lens cleaner, which is often made with alcohol. This should also work fine and you can try this again with a clean tissue or soft cloth. Avoid acetone as a cleaner (nail polish remover), this is bad for lenses and coatings.
In both cases MAKE SURE THE LENS IS CLEAN! Any dirt or fine grain of sand or salt on the tissue or lens will cause a scratch when you wipe the lens, so be careful. For more details, there's also a page I found:
http://www.bobatkins.com/photography/tutorials/lens_filter_cleaning.html
Best of luck!
