The tentacle operates a lot like spiritual weapon, especially in regards to it not being able to be attacked or damaged.
Both the UA version (the Lurker in the Deep's Grasp of the Deep feature, from UA: Sorcerer and Warlock) and the version released in Tasha's Cauldron of Everything (the Fathomless's Tentacle of the Deeps feature, on p. 72) say the same thing about the tentacle:
The tentacle lasts for 1 minute or until you use this feature to create another tentacle.
It has no AC and no Hit Points, and there are no other conditions that could banish the tentacle, so, at least to me, it's clear that it can't be attacked - not effectively, at any rate. An enemy might decide to take a swing at it but it isn't going to have any effect.
You can further compare its description to something that can be destroyed, such as the Echo Knight fighter's Manifest Echo feature (EGtW, p. 183), which says:
This echo is a magical, translucent, gray image of you that lasts until it is destroyed, until you dismiss it as a bonus action, until you manifest another echo, or until you're incapacitated. Your echo has AC 14 + your proficiency bonus, 1 hit point, [...]
This description contains everything you need to know about how the Echo can be destroyed and serves as a good counterexample.