What is the difference between interbank FX rates and open market FX rates?

None essentially. The mid-market is derived from open-market rate.

The open market rate is the rate (buy/sell) quote that you get. The interbank rate is the ‘mid-market’ rate between the open-market rate.

This video explains what the mid-market rate (also known as interbank rate).

https://www.youtube.com/watch?v=...

The mid-market rate is also known as the interbank rate. As it sounds, this is the rate that banks will use if they sell currency to each other. It’s figured out by taking the midpoint between the buy and sell rates used on the open market. This is the only real exchange rate. Anything else is simply a number made up by the banks, including their slice of profit.

Source: What is an exchange rate?.