What do you call this in the picture?

And how is that picture an example of sleep paralysis but not nightmare?!

Curmudgeon, The reason why it is called sleep paralysis is because He/She thinks that a monster is really trapping him/her, preventing that person from sleeping to get out of bed. Therefore, he/she is "paralysed", meaning that the sleeper (person sleeping on the bed) cannot move or anything because the sleeper is scared about what happens to himself/herself if he or she does anything wrong which angers the monster

very interesting. thx for sharing!