Hello, if it has not eaten any food since you got it, and the other one has, I would say there is some issue with this clownfish that is not a contagious disease and not related to water quality - disease and water quality would affect both fish the same way.
This looks like it is a wild caught clownfish. Many wild caught clownfish are damaged during the process, and just do not thrive. Can you contact the store that you bought it from?
I might be wrong, in that case, we need more background information about the tank and the fish. That is explained in this post:
Aquarium Fish Disease Problem Solving Determining the best course of action for a problem with your aquarium can be a difficult task. Even advanced aquarists will often need to seek the help of others for complicated or unique issues. Our intent here on Reef 2 Reef is to offer expert advice...
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Jay