Slacks made his only appearance in the episode "Walk for Your Lives", where the Almighty Tallest checked up on him, and ordered that he do a puppet presentation depicting how he plans to conquer his planet.
When Slacks questions them, he fails inspection and gets a pummeling as a result. He then promises to do better next time.
Facts of Doom
Slacks is the only Invader whose mission is confirmed to still be in operation. As a result, it's unknown when he will conquer his assigned planet because it is hinted he is not a very competent Invader, mostly due to his terrible disguise and the fact that he failed the Probing Day inspection.
His name implies as much, since Slacks means "Lacking in diligence or due care or concern."
In Latin America, his name is "Flojos" which is plural for Lazy.