I had to check back through the code to make sure I wasn't misremembering, but there are no gray slimes in this game, only spacethumper 1. The good news though is that in both games the enemies just get harder over time, nothing is gated behind certain upgrades, so feel free to research the better defensive buildings to deal with them!
Well, with that knowledge (and being able to build the energy shield's whenever the little slimes that deal like a quarter of the health of the fortified walls), I finally was able to get past the block. That and I just moved my base away from the cave. Still excited to see what else is in store for this game (as I do enjoy this game :D)