It is far debateable when it comes to a sheet mask. Some would sheet mask before moisturizer, and some would prefer doing it after moisturizer. Honestly, there isn't a 'you-must-do' answer. Either works perfectly fine; however, these 2 different routines do deliver different effects to your skin.
1. Sheet Mask Before Moisturizer
With this routine, you are replacing the serum step with a sheet mask. Moisturizing afterward will seal and lock in every active ingredient.
2. Sheet Mask After Moisturizer
With this reversed routine, you are using the sheet mask as a barrier to penetrate everything you previously layered underneath. By doing this, you will maximize the effect of your moisturizer
Don't sheet mask longer than directed. As it dries out, the sheet reabsorbs the nutrients back that first offered to your skin.