You can tell it's CG because the glow is directly around the creature, and it's uniform, like it's overlayed on top of the 2D image on screen. If it was really glowing, the water would scatter some light, and it's own glowing form would occlude various parts.
Creatures that glow also tend not to glow uniformly, having cells in certain parts that glow.
You would think that until you realized that alien glow does not reflect off water and it is generated in a uniform glow intentionally. This way when the aliens get caught on film you will think its CG. Its the perfect camouflage!