The question of what is symbolist and what is decadent is complex. The terms were often applied indifferently to the same painters. One way to make a distinction would be to say the symbolism pertains to method and decadence to subject matter.
The decadent themes tended to be either morbid or erotic -- and sometimes both. They were often based on unreal or historical settings -- medieval (or pseudo-medieval), dream landscapes, or classical antiquity were common.
Symbolism refers to the habit of representing things by suggestion rather than realism, with the outer form, as in the Byzantine icon, there to suggest an inner truth. An important connection is that the poet Jean Moreas who wrote the Symbolist Manifesto was himself a Greek (brought up in the orthodox church) who had migrated to Paris and wrote in both French and Greek.
In terms of technique, it is so varied that it is hard to generalize beyond saying that both movements were marked by experimentation and a move away from strict realism.