What is this accrual interest in my tiger broker account for my currency SGD?

Interest accruals occur when you utilize the margin facility of your brokerage account. This applies to all brokerage platforms which offer margin.

You may want to check if your trade value had exceeded the amount deposited into your account. Another scenario I have heard where users accidentally accrue interest is when they withdraw their money before the currency has fully settled after 2 working days.

If you would like to prevent this from happening again, you should contact customer service to downgrade your account to a cash account. However this will remove the capability for you to short stocks and buy/sell options.