Like Tillie, I thought you were asking about a detachable tube that works with a button. For those who have a detachable tube, I removed the dark mould spots by pouring boiling water down the tube and rubbing with my fingers. The silicone material seemed to handle it well. I did this once a month. I also kept the detachable feeding tubes in the refrigerator after each feeding was done, which further prevented the growth of anything.
For tubes that are attached, I wonder if letting a little pineapple or papaya juice sit in the tube for a while would help dissolve any gunk?