It's a nice concept from what I can gather. A... uh... slime, seemingly harmless, bursts out and attacks you with a mech. I just (hopefully) turned your frown upside down !
Oh I like the design, some for a new spriter it's actually quite amazing. It could use some more shading and possibly just actually normal the mouth out a bit but pretty cool.
good first try! nice concept also. slimes are squishy, the mech will make it more of a threat. maybe if you leave them alone too long, the slimes on your planet will build mechs?
Well I used Piq, which is a free online program for pixel art. It seems to be more simplified and whatnot.