If you want close the incoming message from Magento admin, the best approach would be to mark the message as read. You can do this by following these 3 steps below. Once the message is marked as read, the pop-up will not show anymore until a new message is received.
Step 1. Close the message
Step 2. Select See All messages from Dashboard
Step 3. Mark the message as read
Note: If you want to disable all notifications in your store you can do this by disabling the native “Magento_AdminNotification” extension installed on your store. Please note that this is not recommended as you will not receive any notifications at all and will also miss critical security messages from the Magento platform.
Note2: If you want to disable only WeltPixel notifications, and keep all other notifications, we've added an option for this starting with version 1.7.1 - July 12, 2018.
Go to Store -> Configuration -> WeltPixel -> Developer -> Notifications and enable or disable notifications from WeltPixel. Please note that we employ this functionality to announce important product and security updates and disabling product update notifications may expose you to risk of using out of date / unsecured products.