Saw a dot of rust on the bonnet of my car. Help!

I have Grand i10 NIOS and today I saw a rust dot on it's bonnet. I don't know how it happened ? What can I do to fix it ? I also took my car to my local mechanic and he said just leave it and it's not a big problem but I don't like it since I have seen that there's a dot of rust. Also since it's a white car so it doesn't look nice. Please help me guys how can I fix it ? And please tell me ways to avoid such in future.