Yes, most cukes release toxins when they die, but as to how harmfull this is to your tank it depends on what type of cuke you have. I havent heard many horror stories when the cuke in question is the sand moping kind, but the sea apples are definitely known to take out entire systems upon death, im sure that goes for most of the really colorful ones. In nature brightly colored organisms usually stand for posionous. HTH