There's all sorts of ins and outs to this sort of thing - lawyers will tie you in knots over it - but as you possibly have something of value to both you and them, it would be best to get a written agreement from them BEFORE you start doing anything with it with your new employer.
It could be as simple as describing what it is and what it does and getting them to sign to say they recognise this was pre-existing copyright owned by you to which you hold full rights. If they won't do that, you'd best not use it or show it to them.
One niggle - if you developed the software whilst being employed by your ex-company in their time and/or using their resources, they may have a claim to it, not you. It depends on the circumstances and what is in your contract of employment.
(It would in the UK anyway - I believe not all workers in the USA have contracts?).
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