I agree. No worries. It sounds like you have good activity in your culture if it pushed the top off. A few local bugs can't compete with that. If that was a concern, you'd never be able to open your container, right? Also, there is a lot more yeast and bacteria in the flour you feed your culture than in the air.
Ischia is a strong, stable culture that has been around for a long time. This suggests it is very resistant to invasion.