Due to existing limitations, your remaining credit does not appear yet in the Costs & Usage dashboard. However, you will not be billed for the first $800 of Multiplay usage on your account. This should show up when you are billed, so you should not see money taken from your account.
You can see the status of your current New Joiner Credit via cloud.unity.com > Game Server Hosting > usage.
If you see money taken from your account and you believe that you should still be in your credit, please let us know and we will investigate further.