It depends on the law in your jurisdiction and, potentially, on promises you may have made in your employment agreement. Many jurisdictions will allow employers to enforce an IP agreement which you made as a condition of your employment, and employers in both these and under jurisdictions at least theoretically state claims to such IP in a lot of employment agreements or in appendixes to them. (e.g. buried in the Company Handbook which is incorporated by reference, which may or may not be enforceable in your jurisdiction.)
In general, I would tell employers about that sort of thing and ask for a quick two-line email that they have no objection to $NAME_PROJECT_HERE.
P.S. The right time to do this was before starting work on the app, since your options are very limited if they say "Cool, we own that now, any questions?"