If you want to make any control transparent in LUA, use SetAlpha(value), where value is number between 0 (transparent) and 1 (fully opaque).
lua Code:
QuestLurkerBG:SetAlpha(0.3)
Or you can completely hide control using SetHidden(true).
lua Code:
QuestLurkerBG:SetHidden(true)
If you want set transparency in XML use alpha="value". This example sets transparency for backdrop to 30%:
lua Code:
<Backdrop name="$(parent)BG" inherits="ZO_ThinBackdrop" alpha="0.3" />
To completely hide control in XML use hidden="true":
lua Code:
<Backdrop name="$(parent)BG" inherits="ZO_ThinBackdrop" hidden="true" />