decrease, subtract, diminish
减 centers on making something smaller in amount or intensity: decreasing, subtracting, or diminishing.
减 is the simplified form of 減, replacing 氵 (water) with 冫 (ice). The original 減 combines 氵 with 咸, which likely contributes the sound and the idea of lessening. The modern 减 is a visual variant used in some character sets.
The left side 冫 (ice) suggests shrinking or reducing, and the right side 咸 looks like a container being emptied. Picture ice cubes melting away, decreasing in size.
For ゲン, imagine a genie granting a wish to decrease your workload: genie -> ゲン, and the genie subtracts tasks from your list.