In bowl of stand mixer, stir together tangerine mousse mix and milk. Scrape down sides and bottom of bowl.
Using whip attachment, mix at low speed for 1 minute. Scrape down sides and bottom of bowl. Whip at high speed for 8 to 10 minutes or until peaks form.
Using rubber spatula, fold in whipped topping until combined.
Cut sheet cake in half lengthwise. Trim each half so it fits into the bottom of a full hotel pan (approx. 20 x 12 x 6 inches/50 x 30 x 15 cm). Set halves aside and reserve trimmings.
Layer in pan:
1/3 prepared mousse mixture
1/3 mandarin segments
1 piece sheet cake, press down gently
½ reserved syrup
1/3 prepared mousse mixture
1/3 mandarin segments
1 piece sheet cake, use trimmings to fill gaps
Remaining syrup
Remaining mousse mixture
Refrigerate covered overnight.
