It's just algae and debris that collects over time. You can either replace them if it gets too bad, or find a way to clean them out -- There are brushes that they make, designed for that task. Try bigals or one of the other places. It is a good idea to do something about it as it can restrict water flow once it collects enough.
If its just in the hose nearer to the top of the tank its algae. Mine have some green and brown spots near the very top - were light hits it. I suppose if the hoses were opaque black or something then algae wouldnt have any light to feed from.
Your LFS will carry flexible brushes designed to clean inside of tubing. To clean my XP3 hoses I tie some strong fishing line to one of the brushes, a small weight to the other end of the fishing line. Drop the weight down through the tube then pull the brush through with the fishing line. Repeat a couple of times and your hoses are almost like new. Takes 10 minutes every 6 months or so.