How can I report my mom's husband to immigration? He is working under someone's identity. He gets credit cards under his legal name. How is that possible if he is not a citizen?

This is easy enough to do by calling the US immigration number and reporting; but you must have your facts straight before you do this. Are you absolutely sure this is happening because it will effect your mother if it is.

How do you know he is working under someonelse's identity; credit cards are not a problem, anyone can get them. How do you know he is not a citizen?

Before accusing someone you are not sure of, especially a family member, you have to make sure it is all true.

Immigration fraud numbers are easily found on Google or in your local telephone book. How do you do it? You call the number and make your accusations.

Foreigners can open bank account in US, so don't jump to conclusion that the guy is doing something illegal unless you have strong evidence.

If he is using someone else's identity, report this to the police and let it go from there.