The Aberrant Dragonmark feat states:
You learn that spell and can cast it through your mark. Once you cast it, you must finish a short or long rest before you can cast it again through the mark. Constitution is your spellcasting ability for these spells. When you cast the 1st-level spell through your mark, you can expend one of your Hit Dice and roll it. If you roll an even number, you gain a number of temporary hit points equal to the number rolled.
How long do those HP last?
The Freedom of Movement spell states this in it’s description:
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.
A Gibbering Mouther has an ability called Abberant Ground which states this:
The ground in a 10-foot radius around the mouther is doughlike difficult terrain. Each creature that starts its turn in that area must succeed on a DC 10 Strength saving throw or have its speed reduced to 0 until the start of its next turn.
That ability doesn’t appear to be magical. Does that mean that Freedom of Movement won’t stop a creature from being reduced to 0 speed if they fail their strength saving throw?