The simplest way to remove the captured effect is to max out the slave's stress and end the day so they have a mental breakdown. Just make sure they aren't low HP. Later in the game, the dominate spell is quite effective at removing the captured effect. If you prefer waiting, then put a brand on them, upgrade the jail with incense, and assign a confident jailer.
Captured slaves are quite useful even with the captured effect, but you need to remove it for many quests or places to sell them.
Fluid substance is acquired from having sex with a Slime, Nereid, or Scylla (they need at least 1 magic affinity) or defeating ooze in the marsh or sea.