"Cloud" is a prettier image. "Swarm" creates the impression of insects acting with intent, like an army.

Both phrases refer to a group of thousands of moths, flying together. A "cloud" of moths emphasizes that they look like a dark cloud. You see the irregular shape of the whole group, not the individual moths. "Swarm" is used to refer to a group or crowd of animals--sometimes people. It means they have come together into a tight, dense group and are all moving the same way. Some animals do this as a natural part of their life cycle. "It is the time of year when the sparrows swarm." "I saw a swarm of ants in our backyard." Grunion, a kind of fish, "swarmed onto the beach, and people caught them in buckets."

Hi! You really don’t say a “cloud” of months. You can say “a bunch” or you’re correct to say “swarm” too