It looks like a plastic sleeve covering the top of the shock, and it's split down the side, is this a concern?

 

It's a sleeve to help keep grime, grit, salt, and water off of the shock. Is it important? Yes, in the sense that less protection most likely leads to faster shock degradation.

It's very easy to remove a rear shock from a Fit and replace this part. Or just slap some "As Seen On TV!" Flex Tape on there and forget about it.

 

I agree. Tape it up.