It's just gunk, algae, etc building up in the inside of the tubes. It happens to my xp3 all the time, there's nothing to worry about, there's barely anything in your tubes so you should be fine. You can easily clean them out be running a towel through the tubes. I did this by untwisting a metal coat hanger, tying a rag onto one end and just pull it through.
When i clean the insides of my filter tubes, it's amazing what i difference it makes in the amount of water it puts out. It never seems like a thick layer, but it pumps a lot more water once i give it a good cleaning. it's a hassle though, so i don't do it everytime i clean the filter.
Adrienne!
2.0 RES: Turtley and Samson
1.0 Lutino Cockatiel: Lilly
assorted fish, hermit crabs, bugs etc.
If you wait long enough, the debris will build up to a point that the tubes will start cleaning themselves. I've cleaned the tubes using a small brush tied to a string that I pushed through with a dowel. The flexible brushes weren't long enough to push through the tube.
"You become responsible, forever, for what you have tamed." -Antoine de Saint Exupery-
I'm sure if you went outside and if you have a pressure washer you could wash it right out, but beware the gunk will go flying everywhere. The hanger method that I use works pretty well, the pictures you provided show extremely minor gunk, you should see my tubes sometimes....hehe yeah I'm lazy