Just a guess
My guess is that it is some sort of gasket that goes between the spoiler and the rear deck. I don't see it here in the parts list. it's possible that it's already applied to the spoiler prior to installation. Perhaps someone else on this forum can provide some insight.
http://https://www.hondapartsnow.com/Page_Product/SearchByName.aspx?Vin=&Make=Honda&Model=Crosstour& Year=2015&Submodel=&Filter=(bt=3%2c466;et=2%2c9)&K eywords=rear+spoiler
Greater Indianapolis, Indiana (USA)
2015 Honda Crosstour, EX-L AWD; No Navi
LED headlights, LED interior lights
POR-15 painted calipers (red)
StopTech drilled and slotted rotors
Acura TLX rear sway bar (20mm)
Energy Suspension polyurethane sway bar bushings
KartBoy solid rear sway bar end links
K Sport Pro coilovers; lowered to 29"
SPC Performance 3 piece adjustable rear lower control arms
Genuine Honda JDM badging front, rear and steering wheel