It may be that Mickey is one of the bettas who will not tolerate tankmates. How is the toward the Cories? He was the same as most bettas and not used to tankmates and he may not take to them at all especially smaller ones that he thinks he can bully. So you may have to remove them. Was he the first one in the tank? Sometimes the others have to go in first and the betta last but sometimes this will not work. Sometimes if you take the betta out for an hour or so and rearrange the decor and put him back it will make him think he is in a different tank and he needs to find a new territory with new tankmates and it is their home before his. This will sometimes keep them from being naughty but not always. But if you do not have a place or do not want to move the tetras, you can try this.
Just remove Mickey to a small container for about an hour and then after you have moved things about in the tank a bit, you can put him back and hope for the best. If that does not work then the only hope will be to either move Mickey or the tetras.
Hope he will behave one way or the other. Sorry he is being a bully but he is very young yet and does not know how to play well with others.
When my Betta, Fred, first met his tank makes, he did pick on them somewhat, but mostly just displayed to them. What I did was to set up the plants in the tank so that Fred did not have an unobstructed line of sight through the entire tank, which allowed the other fish to swim behind the plants to hide. It worked pretty well for me and Fred soon got used to seeing the other fish but without having an option to pick on them continually.
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