I think they work well as aliens, but aliens can be called monsters too, so it's up to interpreation and most importantly, context. If you put them in a setting that is alien (like quite obviously space), they will be regarded as aliens.
I have a hard time choosing in the first poll so I'll just give you my thoughts. I think you have a nice idea going on, they're very minimalistic and that's a good thing if you want easily recognizable characters, and they're different enough from each other that you can tell them apart. But in my opinion they are a bit too minimalistic.
What i'd suggest you'd do is continue their ongoing theme I'm interpreting, one is pointy, and the other is round. Maybe add more spikes to the left one, or make it longer? And maybe the other should have a rounder body? It just needs to be little things to make them pop out more. I also feel like the left one is already more stylized than the right, so it makes it look more important than the other. The right one makes me think of plankton, for example.
I think you're well on the way there, though! You just need to work on refining their characteristics, give them some time to really think about what works, and try several different designs. Keep it up!