This fluid coming out is semen .Semen, also known as seminal fluid, is an organic fluid that contains sperms. It is secreted by the sexual glands.
Semen coming out at night is also called Night fall or Nocturnal emission or Wet dreams.
Do not be tensed about this issue. It is normal at your age. It is not an illness but a natural phenomenon by which old sperms come out and make space for new sperms.
New sperms are produced continuously in our body by pushing away the older ones.
It does not cause any physical harm or weakness.