Pickled Puss Alien Scare Illusion
This idea is based on that very old magic trick called the mirror glass. This is a picture of a cheap one:
You can see an average performance of it here:
The mirror glass works using a flexible double-sided mirror that fits inside the glass. Looking at it dead-on you can see it at all.
My idea was to use this on a much larger scale, and not use the trick part at all.
There would be an actor submerged in a tank that barley fits him. His headless body just floats there in the cloudy green water. When people come by, he springs to life pounding on the glass and kicking.
In the drawing, the actor wears an alien water proof costume. People won't be able to see the back. The actor's head is safe inside a secret compartment behind a short 2-way mirror. The 2-way mirror lets the actor see who is walking close but they won't see him. Water will not fill up his head area. That area should be secured to the tank from the mirror to the glass sides. The costume needs to be a little larger at the shoulders to let the actor's shoulders stick out the back. The cut off part of the neck is part of the water proof costume.
I added air jets at the bottom in case you want to show bubbles.
There is light at the foot of the tank to show the alien in a very creepy way.