It's a level 1 spell, and the fact that it doesn't say that it can be set on fire, tells you that it can't be according to the rules. Otherwise you have a level 1 spell that is far more powerful that its casting level.
I mean, it's not really too powerful? As I said, anyone with a speed of 20-ft can get out of it on their turn. So you'd have to cast grease and make it flammable before the enemy gets their turn, which costs 2 actions. 2 actions for grease + idk what, 1d6 fire in a 10-ft square? Dex ST to negate the fire? Plus I'd probably rule that the grease burns up in 1-2 turns instead of lasting for a minute, so it's really not that powerful. It'd be ~equally as effective to just firebolt the enemy if they didn't fall prone or just attack them with advantage if they did.
Edit: I acknowledge that Jeremy Crawford says grease can't be lit on fire, but to that I refer you to my above image