That mottled look is often a sign of insect damage - specifically spider mites. They're very common problem, on house plants because they love warm, dry conditions.
Check the backs of the leaves with a hand lens or just by running the leaf between your thumb and finger. If it comes away with a rusty stain on your finger, you have spider mites. Also check for their tiny webs on the backs of the leaves.
Treatment is a thorough spraying with a mild soapy water solution, about 1/2tsp dish soap or liquid hand soap to a quart spray bottle of water. Spray to the point of dripping. Be sure to get the undersides of the leaves, the stem and the soil surface, as well as wiping down the pot. Repeat the treatment again twice more in the next week or so - very important to repeat because there will be eggs left behind that will hatch, and some bugs will hide well enough not to be killed.
To prevent their return, a nice warm shower of water once a week (in your bathtub or shower if possible) will keep them away. The plant will love this treatment, too.
These plants grow outdoors in blazing sun here in Florida, so the more good light you can give it the better.
