Thrive used to have a console, accessed by pressing the tilde key, but it was buggy and you couldn't really do much with it. Disco mode was in the April fools joke that was version 0.4.0; it replaced all nuclei with flashing disco balls and had this theme playing nonstop in the background. Disco mode isn't present in any of the serious versions though.
I think the devs might be planning to re-add the console eventually, and have Disco Mode be accessible through it, or perhaps some other method, such as entering the Konami code.
Nah, it's a Tireeie, just another sapient, technologically advanced underwater species that just happens to also have an odd number of eyes, long, slit-like gills underneath the fin, and manipulators near the mouth! Seriously though, they're two completely different ideas. It's a bit silly how many similarities there are between the two.
EDIT: They're technologically advanced because they were uplifted by an alien race. They were stuck in the stone age for a very long time due to the inability to smelt metal underwater.