I removed a piece of live rock to remove Aiptasia and just leaving it out of the tank for awhile. Any concern about or procedure to follow before placing it back in the tank?
Not if that's your only rock and you have others. Even if only rock with sand and all I doubt it'd cause much of problem. You may seem ammonia spike with decomposing biologics...you may not. And again, if just a small portion of total rock/sand you probably wouldn't see it and if did it wouldn't hit toxic levels.
That's assuming not chem treatments - and then make sure rinse well.