No

Tentacle Slam is not technically an attack.

In the stat block, the other actions include the phrase Melee Weapon Attack while Tentacle Slam has no similar verbiage. Tentacle Slam does not call for making an attack roll with a d20.

Tentacle Slam is an action that causes grappled creatures to make saving throws. It does not qualify as an attack for the purposes of the Otyugh's Multiattack ability.

When an attack is not an attack

This prior answer explains what counts as an attack in the game mechanics. That answer draws upon the passage from the Player's Handbook:

If there's ever any question whether something you're doing counts as an attack, the rule is simple: if you're making an attack roll, you're making an attack (PH, p. 194)

There are many actions that would likely be considered "attacks" in-universe, but which are not attacks mechanically. Examples include grappling and spells with save versus damage effects.

When reading a term in the rules, interpret it with the definitions in the rules. When deciding how NPCs/monsters react to something, use common sense.

RAW, grappling with reach doesn't impose disadvantage

There are no rules stating that being within a monster's reach imposes disadvantage. The rules explicitly say "within 5 feet", not "within an enemy's reach". Grappling, unless otherwise stated, does not naturally impose disadvantage to attacks by itself.

Grappling does say this:

When you want to grab a creature or wrestle with it, you can use the Attack action to make a special melee attack, a grapple. If you’re able to make multiple attacks with the Attack action, this attack replaces one of them. [...] The [grappled] condition also ends if an effect removes the grappled creature from the reach of the grappler or grappling effect [...]

So if you are not within 5 feet, and grappling does not impose disadvantage, and grappling relies on reach rather than adjacency, then being grappled at that extended reach will have no effect to your ranged attacks.

This does bring up a possible question as to how movement/positioning works after being grappled with reach, but that may be grounds for a different question.