We all get called Mommy from time to time but we normally continue to correct it. We aren't their mommies and to be honest, even if their Moms were saying they understood and were okay with it, deep down it has to hurt. Bad enough that the moms have all the guilt associated with working outside the home and not being able to be home with their child in person, no need to rub salt in the wounds.

It's not that hard to change it - like anything, consistency and firmness go a long way to end unwanted behaviours. It doesn't sound like you are committed to changing this, and maybe enjoy it but IMO, it's unprofessional and inappropriate not to correct it every single time.