What sort of materials can I use to make a latex mask look wet without actually being wet?
There are two main things that make objects look wet:


The glossy, reflective finish that water has
The shape and depth of the water - you can see not just the surface of the water, but the surface of the object below it


Anything that satisfies these properties will work.

Given that you probably want the tentacles to remain flexible, I would recommend clear silicone sealant to make the volume of the ooze, adding drops to the end of tentacles etc, followed by a coat of flexible gloss varnish for the reflective finish.