Freedom of Movement (PHB, p244):
You touch a willing creature. For the duration, the target’s movement is unaffected by difficult terrain, and spells and other magical effects can neither reduce the target’s speed nor cause the target to be paralyzed or restrained.
The target can also spend 5 feet of movement to automatically escape from nonmagical restraints, such as manacles or a creature that has it grappled. Finally, being underwater imposes no penalties on the target's movement or attacks.
A grell's Tentacles attack (MM, p172):
Hit: 7 (1d10 + 2) piercing damage, and the target must succeed on a DC 11 Constitution saving throw or be poisoned for 1 minute. The poisoned target is paralyzed, and it can repeat the saving throw at the end of each of its turns, ending the effect on a success.
The target is also grappled (escape DC 15). If the target is Medium or smaller, it is also re strained until this grapple ends. While grappling the target, the grell has advantage on attack rolls against it and can't use this attack against other targets. When the grell moves, any Medium or smaller target it is grappling moves with it.
So if you fail the Constitution save while under Freedom of Movement, do you still get paralyzed by the poison?
Best Answer
Yes, you still get paralyzed
...because the effect causing the paralysis is not magical.
To determine whether an effect is magical or not, we refer to the Sage Advice (page 17) ruling on a dragon's breath weapon:
The answer to each of these questions is no, so the paralyzing effect of the grell's poison is not magical, and is therefore not prevented by Freedom of Movement.
That being said, the spell is not totally useless against grells as it enables a victim to use 5 feet of movement to automatically escape from a grell's grapple.