Or vice versa: It's plant DNA that has infected an animal and uses it for reproduction. The algae's chloroplasts haven't figured out how to survive the step into the next generation, yet, though.
I recall a definition of mental illness that requires it to have a negative impact on the person.
A mutation may be adaptive or maladaptive. It depends on the creature as a whole - and also the current environment.
That environment is analogous to the market part of "product+market" for a startup: the identical technology/product can be a winner or a loser, depending on extra-product factors.