The reason the Discovery harnesses the Tardigrade is its ability to travel along the mycelial plane, and thus transport the ship at faster than warp speeds.
Now these two creatures appear to have similar, but not identical characteristics. Keeping in mind the Nacene appears to have better than human intelligence, whereas they were only just confident the Tardigrade was intelligent at all. They're both aliens that can travel at faster than Warp speeds and survive in the expanse of space. (They also both have a blue-glow).
Indeed, the following commentator writes:
Is there a relationship between the Tardigrade and the sporocystian life form from Voyager?
My question is: Is there evidence for a common ancestor between the Discovery's Tardigrade and Voyager's Nacene?