I don't know if this is related to marijuana.

Sensed Presence is very common during true sleep paralysis (enough that it's called the Old Hag), and sensing such a scary presence but then trying to move, finding you can't, and focusing on the scary 'thing' can contribute to that fear I'd try either mentally preparing yourself (If I see something scary, it means I'm almost in a dream! Don't give up!), getting in the habit of ignoring whatever you sense (hard, I know) and thinking 'happy thoughts' to distract yourself and maybe direct any hallucinations (if possible), or preparing a few of the techniques that a few people have listed as an aid to getting their body out of the paralysis... I've read that wiggling the extremeties and opening and closing the eyes, if possible, will sometimes do it.