Sleep paralysis is something else, a natural state where your body keeps you from acting out your dreams; if you don't have it, something is wrong with you; you should not be sleep walking, running/kicking/fighting, etc. while in bed (my husband accidentally kicked his wife one night, when she was pregnant!) etc.

It's related to dreaming, as that's what it is there to protect you from, acting out your dreams, and sometimes one can become semi conscious while the body is still in that state and that can be scary. But if you are having a nightmare and can't run/scream, etc. from ghosts, etc. that is a bit different.

"Symbolically, dreaming that you are paralyzed means you are feeling helpless or pinned down in some aspects or circumstances of your waking life. You may feel unable to deal with a situation or change anything. Alternatively, the dream suggests that you are feeling emotionally paralyzed. You have difficulties expressing your feelings."

"To dream that you are immobile signifies feelings of being trapped. Alternatively, it shows that you are too rigid in your attitudes and decisions."

From: dreammoods.com dream dictionary