A subspace being appearing as Silva La Forge
The Marijne VII subspace beings were a non-corporeal species inhabiting the area around the gas giant Marijne VII.
In 2370 individuals of this species caused accidentally the death of the seven crewmembers of the Federation starship USS Raman who were investigating this area.
Lieutenant Commander Geordi La Forge connected himself with the interface probe and was able to communicate with one of these beings which appeared as his missing mother Captain Silva La Forge. Geordi first believed that his was really his mother talking to him but learned the true identity later when this individual attacked him. (TNG: "Interface")
The individual appearing as Silva La Forge was played by actress Madge Sinclair.