I didn't notice this discussion till now. I never really studied much about optimizing. I usually just kill off particles, lasers, tractor beams, etc when the player finishes a level so there's less working elements throughout the map. In this case, I used occlusion so as not to draw elements in the next room till you get there. What you're seeing is the transition when you walk past the occluder. When I play tested the map, this didn't show at first. After, when it did show, I thought about ways to hide it but it would have made for more work, so I left it.