It's Xander the Monster Morpher: Universe Breaker's 1-year anniversary (Steam version)!  To celebrate, download version 2.5, and for the entire month of August, normal Slimiuses have a 50% chance to be Anniversary Slimiuses!  

If you use the attack Flame Gatling while morphed into an Anniversary Slimius, the flames will be a bright rainbow of colors!