imageGaster's pirate ship in a body of water with his crew.
imageKris without soul for a long time.
imageMoss.
imageHometown Lake which is a body of water.
imageKris and Noelle drowning in Hometown lake.
sticky-yellowAll of them are green.
imageMoss near a body of water.
sticky-yellowKris might've turned green when drowning themself at the lake because they're moss in human shape with a soul that turns to moss after a long time without soul. And if they get a soul just at the right time — they turn green permanently.
sticky-yellowThe same thing might have been happened to Gaster too. When he was sailing, he fell off the ship's deck and because he already doesn't have a soul, he "instantly" turned green but was rescued by the crew just in time for him to not turn to moss.
sticky-yellowGaster might've been sailing in the Hometown lake when that incident happened.
sticky-yellowAnd, by eating the moss ic Chapter 1, they continue the moss lifecycle.
imageKris Mitosis.
sticky-yellowKris might be a moss because they are green when they don't have the soul for a long time, and according to the mitosis theory, they multiply by mitosis. And which thing does mitosis? Moss.