Clotet has claimed Bela was targeted while he was receiving treatment for an injury 16 minutes into the Championship game.
"I heard a comment, I followed the protocol and made communication with the fourth official," he told BBC West Midlands.
"I want to state clearly that it was one individual and I know clubs are working hard to get rid of this problem.
"At the end of the day it's not the clubs, it's not Millwall, it's just one individual. But we have to make people understand that it is not acceptable in our game."
Millwall manager Gary Rowett said: "The official said to me he hadd heard a comment. The difficult bit is almost as though there is definitely guilt there because it is Millwall.
"I understand it and it has to be proven and if it is proven then I think we all agree that it is not something you want here or at any football ground, or anywhere for that matter."