Why is my account equity displayed in C$ when my trading accounts are in US$?

Your equity display currency is based on the base currency of the first trading account you created. 

For example, if your first trading account was created in Canadian dollars (CAD), your equity may continue to be displayed in C$, even if that account has since been closed and your current trading accounts are in US dollars (USD). 

This is expected behaviour. It does not change the base currency of your current trading accounts or affect your trading, balances or transactions.