Bluegill

Neural parasite (or bluegill) aliens attempted to infiltrate the Federation's Starfleet Command by physically infiltrating high-ranking admirals within the organization. Their plans were thwarted by Captain Jean-Luc Picard and Commander Will Riker. However, the mother creature sent a message which Data identified as a beacon, indicating that their return would only be a matter of time. 

One of the bluegills had infiltrated Teero Anaydis. This allowed him to communicate telepathically with the Spawn Mother. Presumably, the bluegills were using The Circle as a cover for their own activities suggesting they had overcome the weakness of not knowing about their hosts' pasts. ("Divided Loyalties (LP episode)")

Star Trek: Hidden Frontier
The Ethereals used these neural parasites as a means of recruiting new soldiers into the Grey Confederacy.