Slug is a Red Triangle Slug that is completely red so the triangle part can't be seen anymore...
Um.
The pinkish part is a flap of skin known as a skirt on a slug.
The two black dots above Slug's mouth is actually a second set of tentacles that are retracted. They can stick out like her eye-tentacles but not quite as long.
They're not really a second set of eyes though... They kind of feel and smell things. Nose-Tentacle type of thing. Lol.