This is a self-shadowing error that can result from low tessellation or when objects do not have thickness. It usually occurs in concave areas or when the light is behind the polygon mesh. In these situations, it can introduce light leaking.
There are some workarounds:
- Increase the number of subdivided polygons (the artifacts will become smaller).
- Give the object some thickness so that it is not a single sheet of polygons.
It is not possible to make Arnold lights visible to the camera. However, the Mesh Light does have a 'Light Visible' option.