Why am I being charged 9.99 a month?

Unknown Charge

• Queer Charge? --> If you see an 'itunes.com/bill' or an unfamiliar charge on your statement
• If you have an issue with an app on App Store or billing --> Request a refund for an App Store or iTunes Store
• You can also request a refund for a recent renewal of an in-app subscription.purchase 

Phishing

• Beware of --> Phishing & Other Suspicious Emails 
• Know how to --> Identify legitimate emails from the App Store or iTunes Store 
• Avoid phishing emails, fake 'virus' alerts, phony support calls, and other scams —> Use these tips to avoid scams and learn what to do if you think your Apple ID has been compromised.

Did you by any chance sign up for an Apple Music subscription? That costs $9.99/month. See this link on how to check your subscriptions on your iPhone --> See or cancel your subscriptions - Apple Support

There's your answer. The app appears to be free, but as with most apps, there are in-app purchases and you evidently agreed to one of the monthly purchases, which was one month free. If you don't want it use the link to cancel the subscription I provided. You will be charged for the month you agree to pay for, but you won't pay for future months.