Hi truefalcon, hope this is what you're looking for...
Click on the specific sprite instance that you want to be made transparent.
Look in the Properties bar, under the top (Object type properties) section and note the number against the UID.
Then create an event, pick your sprite and click on "Pick instance with UID", then type in the UID number you noted.
Lastly add your action (check out the Appearance group of actions for your sprite: Set Invisible or Set Opacity)