Return pump and water changes

When doing a water change I have to shut off my return pump in my sump as the return chamber is small and is the lowest section in my sump or it would run dry if I took out more than 2-3 gallons. Anyway, when I shut the pump off I am getting lots of slivers of a "film" like build up (which I assume is build up on the inside of the return plumbing /pump) back flowing into the return chamber. Is this normal? It's a 65 gallon tank which is only 2 months old, will this go away? I just siphon it out as I'm doing the water change, but just wondering if its preventable, maybe I just need to pull the return pump out for a cleaning. Any advice or experiences would be appreciated. My searches turned up nothing.

No it doesn't look like gas on water, looks like the slime coat that develops in pipes / tubing, I will take a picture next water change I do.

Yea it's kinda slimey, it makes my return chamber look like a feather pillow exploded under water, hundreds of little slivers of it everywhere.

It's build up from inside the plumbing and rather normal. Not much can be done to prevent it. In our previous set ups, I wouldn't even take the time to siphon it out. In our current set up, I can perform a 60G water change without shutting off the main return so it's not an issue.