Play along? Anything with a spine can experience a spinal injury, particularly if it is a creature prone to quick movements and that leads with the head. Since the lineatus wrasse, and all fish falls succinctly in the Phyum Chordata they have spines. Observing spinal injuries in the other Phylums of birds, mammels, amphibians, and reptiles, spinal injuries exhibit in a number of ways but similarly throughout the phylums. What do we notice? In each of these groups there can be trauma that causes limited to zero mobility behind the injury, as the mode of the brain communicating with the muscles is damages or severed.OK I'll play along. What exactly can one do for a spinal injury? Feed antibiotic laced food to possibly prevent any correlating internal infections? Just hope it heals on it's own? Is the condition usually terminal?
Just like in people, and other chordates there can be a wide range of severity. Some cases healing over time, others never healing.
The biggest issue in the case of fairy wrasses is effectively getting enough food, as the swimming is severely affected. But if it gets enough food and the injury is not too severe it is possible for it to make a full recovery.
No further treatment is needed, it is either severe enough that it won't heal, or not that severe and the fish will recover. I have seen it with dartfish, tilefish, fairy and flasher wrasses. In unscientific observations I have seen about 2/3s recover fully in a few weeks.