FOM, since it hasn't yet been FAQ'd (why we'll never know), is a spell that is routinely abused by players.
They often take the wording and spin it to mean ANYTHING that impedes movement doesn't work.
By that logic, a human gets a leg chopped off still walks as normal because the dismembering "impedes movement."
They sometimes argue that's going too far, but then explain mental effects also are negated by FOM. For instance, any condition which impedes movement. Just ridiculous and needs FAQ'd so players will now. I know it's up to us GMs to rule this, but at least there'd be something to fall back on.
It would seem that spells which target an individual and specifically impedes movement by specifically calling out movement or speed would be what FOM protects against. Icy Prison is a doozy, though- it is in fact a targeted spell.
Does FOM allow unfettered movement under water? I think there's universal agreement it does- so what's the difference between water and ice? Nothing except its state- which in Icy Prison's case is solid. But water pressure and the tension/resistance of water while being under water is having the same exact impediment upon the victim as the ice...and ice is water!