It was the late 1970s. We could get little canisters of "slime" out of the twenty-five-cent gum machines. A dollar's worth of "slime" was enough to fill the monster. It was actually better than name-brand Slime(tm) because it was cheaper and came in a wider variety of colors. Slime(n/t) only came in opaque green, orange or purple. I preferred the clear red stuff.
The game itself was kind of "meh" but I used the slime monster to attack my little green army dudes and Hotwheels vehicles in a little model city.
Man, child molesters had it easy back in the 70's. No online sex offender database, no Chris Hansen, just walk up to some kids at the park and ask them if they want to play the Slime Monster Game (tm).