The wording of the first portion of the spell, "This spell conjures a field of rubbery black tentacles, each 10 feet long. These waving members seem to spring forth from the earth, floor, or whatever surface is underfoot—including water" seems to indicate that you can only cast the spell on a surface that is currently the "floor" based on the local gravity.
That said, if there were no "floor" or "down", I would allow the user to cast it on any viable surface, such as on the astral plane.
The is no mention of the tentacle being foiled by invisiblity or concealment. I would either treat the tentacles as if they had blindsense, or presume there are so many tentacles anything in the area is going to be hit by one.