When you receive an email or text message prompting you about an existent webshell on your server, it indicates that your server has been hacked and a webshell file has been uploaded. The hacker is able to access and modify your website or data in the database.
You can isolate the webshell file through Server Guard. But further troubleshooting is required to figure out the specific intrusion cause. Otherwise, the hacker can still manage to intrude into your system using the vulnerability.