If this spell is cast, and than the slaan decides to forget it - will it still remain in play? Is there any wording on this in the BRB?
I would argue that Remains in Play spells stay in effect until they are stopped by normal means, even when the Slann chooses to forget the spell. This biggest support of this is that when a mage miscasts a spell and forgets the spell he cast (Power Drain on the chart), the spell still is in effect. This is further supported by the FAQ: "Q: Does a remains in play spell end if the Wizard who cast it has his Wizard level reduced to 0 or if he forgets the spell? (p36) A: No."