"From The Fog" operates at the world level in Minecraft, meaning if it is enabled on a server, it should affect all worlds unless specifically configured otherwise. However, since "SimpleWorlds" is a third-party plugin, its compatibility and interaction with "From The Fog" may vary.
To set "From The Fog" to work on a single world while using "SimpleWorlds," you'll want to configure the plugin to limit the datapack's effect to a specific world. Typically, this kind of configuration would be handled in the server settings or plugin configurations. You may need to consult the documentation for "SimpleWorlds" to see how it manages per-world data and plugins.
If you have more specific technical configurations or need further assistance, mentioning this to a server administrator or the developer of "SimpleWorlds" might provide more clarity. Additionally, our <@&1107810118973792266> might also help with specific server configuration issues.