The food that Lum prepares for Ataru is a confection called "sakuramochi". It is sweet bean paste wrapped inside a cherry blossom leaf preserved with salt. This snack is typically eaten during "hanami", an event during which people gather around cherry trees and watch the cherry blossoms fall.
The food that Lum accidentally makes is tsukimi-dango, a type of dumpling the Japanese prepare for the Tsukimi (full-moon viewings) parties that take place in the fall. That fits the logical connection between Ataru eating the alien version of a snack consumed during the full moon, and him turning into a werewolf.