How is its swimming? If it looks as though it is swimming head down it would be a swim bladder infection. If so a sterilized needle can release the gas. If it is an intestinal blockage stop feeding for a couple of days.
Are there any other wrasses in the tank or any other tankmates showing interest in him? If not just give it a few days he should start to get more and more bold. C. rhomboidalis is a pretty passive species.
Though the article cited specified that urchins are part of their diet, they lack the dentition to open the, up, so as long as you dont have triggers or puffers that are able to open them up, the venusta wont eat them. The article also mentioned a 30g aquarium as the minimum tank size, this is way too small for this fish, so that particular article may not be the most reliable.
They are a s,aller species of Coris and relatively peaceful, though I have found them to be a little pushy towards Halichoeres, Cirrhilabrus, and Paracheilinus, but ignores other Coris, Anampses, Macropharyngodon, and Thalasomma wrasses.