Do you mean one of the Subnautica mods from https://store.gx.me/mods/?search=Subnautica&tagAlias=desktop?
If so, you can just goto the URL opera://mods or open the mods sidebar panel and remove the mod. However, the effect that's causing issues with other sidebar apps might a shader from the mod or CSS from the mod. You can turn off just the shader at the URL opera://settings/shaders_settings or just the CSS at the URL opera://settings/web_modding_settings.