Depends entirely on your employment agreement with your employer. Some employers claim everything you create, others only claim the work you did for them as part of your employment. (Also, some jurisdictions prohibit such broad claims in employment agreements, but most don't.)

Best-case scenario: find a prospective employer that actively wants to support your work on your service because it helps them, and work with them to create an employment arrangement that works for you.

Failing that, you should talk with your prospective employer, explain the situation, make it clear that you'll never work on it during business hours or on business resources (unless they explicitly want you to), and get it explicitly excluded from your employment contract.