Now I'm about to make an a$$ of myself, but I'm still gonna do this because I'd like to see the response.
Yeerks are not plants. Yeerks are not animals. This is all conditional on one unproven condition I'll get at later, but if it is correct, Yeerks are not either of these, and-because I know them better-I'm going to use animals to try and show why.
Definition of animal, obtained from merriam-webster.com :
any of a kingdom (Animalia) of living things including many-celled organisms and often many of the single-celled ones (as protozoans) that typically differ from plants in having cells without cellulose walls, in lacking chlorophyll and the capacity for photosynthesis, in requiring more complex food materials (as proteins), in being organized to a greater degree of complexity, and in having the capacity for spontaneous movement and rapid motor responses to stimulation
What's important here is that kingdom Animalia part; the rest refer to characteristics that are generally used in the common notion of describing animals (e.g. complexity is a relative term, and sponges aren't exceeding rapid at most things).
Animalia is an evolutionary group: the things descended from an ancestral organism. It's not whatever traits they do or don't have that places them in the group-that truly make them "animals"-it's a more of a heritage thing.
Point is, since Yeerks are alien, they are not a part of this group-no relatedness to any of Earths creatures, animals or plants, no matter how they are alike.
Same goes for the other aliens.
I think of it easiest with the Leerans: They are amphibians-they live on water and land.
They are not Amphibia (like frogs and newts and such) because they are not an evolutionary relation to the Amphibia of Earth. And unless you are a taxonomist, you say amphibian to refer to Amphibia.
All of this rest on the condition that no one went around planting things from one world on another: a condition unproven and slightly ruptured by the Pemalites. But there's no direct evidence that the anything had a hand in generating any of the intelligent life forms we see in the series in this manner e.g. no skrit na took a frog, dumped it on a new planet, and then got Leerans.
*Except the Iskoort and (arguably) Garatrons (possible Andalite kin)
Assuming this is true, whether or not Yeerks photosynthesize, or have cell walls, or anything else that is plant-like, they are not, in the biological since plants. Nor are they animals
They are aliens, evolved and much more interestingly (imo) arisen independently of life on all other worlds-their commonalities with others superficial and coincidental.
Fascinating sure, but a real mark on the limits of human imagination that any other life inescapably resembles the Earth scheme of plant, animal, and the occasional germs and fungi.