Based on English grammar I would argue that it can't tentacle attack on this turn. The form used in the sentence "if it is grappling a creature" is the present continuous. This form is used for two cases (source):

• activities at the moment of speaking. This is not the case as nothing is grappled at the moment where the attack action is declared. You also wouldn't say "I am attacking" if you are not right now doing it but declaring the intent to do so in the near future.

• future plans or arrangements. There is no arrangement to grappel someone. Also from my understanding this usage of present continuous would require specifying the time when you plan on doing it. "if at the end of the attacks it is grappling someone, it may use its tentacle as well".

Based on English grammar I would argue that it can't tentacle attack on this turn. The form used in the sentence "if it is grappling a creature" is the present continuous. This form is used for two cases (source):

• activities at the moment of speaking. This means when declaring the attack action, something is already grappled. You also wouldn't say "I am attacking" if you are not right now doing it but declaring the intent to do so in the near future.

• future plans or arrangements. There is no arrangement to grappel someone. Also from my understanding this usage of present continuous would require specifying the time when you plan on doing it. "if at the end of the attacks it is grappling someone, it may use its tentacle as well".