Slippery Grease is a well-known invention on The Christopher Show.
The physical appearance of the invention is unknown due to it never being seen, but in the series it is sort of like a running gag (of course it isn't seen again after Season 9 due to Danny not using inventions anymore) like the Invisible Laser Camera.
This invention dates all the way back to Season 2 when Danny put the slippery grease on the stairs so Chris would fall and Mom would think he needs glasses. This very invention is the reason Chris has glasses from Seasons 2 to 6. It appears again in the Season 3 Premiere when Danny implants this in the pool so when Chris goes in he would slip.
It appeared in Seasons 4-6 and then again in 7 when Chris was answering the door in A Piano Tale and he slipped, due to Danny putting slippery grease on the floor.