Oracle Text

At the beginning of your upkeep, you may have Cryptoplasm become a copy of another target creature. If you do, Cryptoplasm gains this ability.

Card Rulings

6/1/2011 You choose the target for the triggered ability when the ability is put onto the stack. You choose whether or not Cryptoplasm becomes a copy of that creature when the ability resolves.
6/1/2011 The copy effect lasts indefinitely. Often, it will last until it is overwritten by another copy effect (if it copies another creature on a future turn, perhaps.)
6/1/2011 If the creature is an illegal target when the ability tries to resolve, it won’t resolve. Cryptoplasm won’t become a copy of that creature; it remains whatever it was before.
6/1/2011 If another creature becomes a copy of Cryptoplasm, it will become a copy of whatever Cryptoplasm is currently copying (if anything), plus it will have Cryptoplasm’s triggered ability.