Yep, the "rotten eggs" smell pretty much nails it. Many decades ago, before the rise (and fall) of the undergravel filter, they called it "sudden tank death syndrome", so it was common enough to be labeled. That was before the gravel vac' as well, and to prevent it, they would stir the substrate with their finger while syphoning mulm from the aquarium. With the introduction of the undergravel filter, water circulated through the substrate, preventing pockets of the anaerobic bacteria that produces the toxic Hydrogen sulfide gas. Now that the undergravel filter is no longer the first accessory recommended for an aquarium, we've come full circle and the issue once more becomes a problem. In a planted tank, the plants circulate enough oxygen into the substrate from their roots to prevent major pockets of anaerobic activity (and thus toxic gases), but areas without such root activity are still subject to the problem. Most of the time, the issue never arises because, although most aquarists no longer use an undergravel filter, nearly everyone uses a gravel vac', so any anaerobic pockets are usually disturbed before toxic gases build up to a dangerous level. The problem arises now because it is not common enough that everyone is aware of it, so when there seems to be a reason to not vacuum the gravel in the usual way (as in your case of not wanting to suck up baby shrimp), the anaerobic condition is allowed to go undisturbed and gases build to the point that they bubble up, "killing" the tank.
I guess it's really a case of "those who do not know history are doomed to repeat it", huh? I know about it from my mom's old aquarium literature, yet it's so rare now that it didn't even occur to me (nice call, jeffkrol!).
But as for removing a film from the water's surface (without a surface skimmer), you can gently lay a paper towel on the surface, then pick it up from the center. The scum or film is trapped by the paper towel and you just toss it. You may have to do this a few times, depending on the size of the aquarium or the amount of film.
"May the Fish be with you."