Clean all the connections and the light itself. it may be developing a film of algae on it. If that doesnt work it is probably faulty
If not take it back LED's should last for years.
Good question. LEDs are good for light but they should be properly sealed so no moisture can creep into the works. Being sealed means bad heat dissipation (yes, LEDs do emit some heat) so when switched off and cooling, any tiny compromise in the sealing will draw the in moisture. The LEDs made for underwater will be totally sealed but, the cheaper ones made with plastic covering don't seem to last too long.