Significantly less macroscopic life. If it's a very small body it is likely to dry up eventually, so that rules out long-lived fully aquatic lifeforms. Even predatory insect larvae (for example, dragonfly larvae) have a much longer development cycle than mosquito larvae, so you won't find them in a body of water that will only be around for a few weeks. The other factor is oxygen... even a still pond that exists year around will have much lower oxygen levels than moving water, and that's a significant limiting factor for aquatic life. Mosquito larvae are air-breathers (they rest just under the surface and have a "snorkel" that breaks the surface to breathe) so they can live in almost completely hypoxic water so long as there is microbial life for them to eat.

He's taking about animal life, not bacterial.

In Rivers there are likely fishes etc, that's not the case with still water, as it often goes bad really quickly unless there is a filter installed, which usually also introduces some oxygen into the water etc.

Another reason why Mosquitos usually only use still water is that the surface tension is necessary for them to inject their eggs, and moving water doesn't support their weight

No, a small puddle is not going to have fish in it that eat mosquito larvae