Every time I boot up my iMac "... is an app downloaded from Internet. Are you sure you want to open it?"
Solution 1:
First place I'd check is System Preferences:
Users & Groups - Login Items.
Scroll through the list of items there, these will open upon log in (which may be part of your boot process), and delete com.aescripts.ZXP-Installer.helper if it is there.
If not there, try looking in the the following directories:
/Library/StartupItems
/System/Library/StartupItems