Slusho was a reference to J.J. Abrams' previous project Alias. It played a heavy role in the viral marketing for Cloverfield. In its original conception, it was a flavored frozen drink produced by a subsidiary of Tagruato, and spelled "Slusho!" (with exclamation mark).
A drink closely resembling how slusho is generaly depicted can could seen on the bar menu, and was presumably intended to depict the drink.