Was he actually 'stuck' under the wood? If so then he is probably just injured slightly and hopefully will recover. Unless you have fallen behind with maintenance, I wouldn't make any water changes as any changes can cause stress. Also, only add chemicals (including salt) as a last resort if you have a specific problem that you need to treat as all chemicals will stress fish and mostly do more harm than good. In addition, most will also damage filter bacteria to some extent, do again, add only if you have to!
I assume those bottom fish are Siamese Algae Eaters and not Chinese Algae Eaters? If they are CAE's, then I think they are the culprits!
I've only kept rainbows once and found them to be very disruptive. They would chase each other and fight and disturbed the other fish in the process. They would frequently gang up and bully one individual, do perhaps that happened here when you weren't watching? I gave mine away.
On a different note, but for that same reason, I don't think angel fish are suitable tank mates. However, that was my one and only experience, so perhaps I was unlucky!