Yes that is the Eradica. It's not supposed to be invisible, there's a glowing blue particle that's supposed to take it's place (see intro Movie).
However, if there is too many particles on screen (I.e. very large battles), sometimes the game doesn't show all the particles. Hence the invisible Eradica.
That said, the invincible part is a normal part of the Eradica's moveset. If you EVER kill a level 6+ Eradica, RUN. The instant it becomes invincible it's already dead, no matter how much health it regains it will be killed at the end of the duration of that ability, so you already won. All it can do is inflict as much damage as possible on your fleet, so don't give it the opportunity and run.
