If you want close the incoming message from Magento admin, best approach is to mark the message as read. You can do this by following these 3 steps below. One than message is marked as read the pop-up will not show anymore until 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 miss also critical security messages from Magento platform.
Note2: If you want to only disable WeltPixel notifications, and keep all the other notifications, we 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 we are using this functionality to announce important product and security updates, disabling product update notifications may expose you to risk of using out of date / unsecured products.