Cannot be counterspelled
The lair action description says
The Aboleth casts phantasmal force (no components required) on any number of creatures it can see within 60 feet of it.
Counterspell description says
Casting Time: 1 reaction, which you take when you see a creature within 60 feet of you casting a spell
The Aboleth is casting a spell, so it could be counterspelled if you could see the Aboleth casting the spell. However, you cannot see the Aboleth casting the spell, because this lair action specifically says no components required.
"[...] To be perceptible, the casting of a spell must involve a verbal, somatic, or material component. [...] Xanathar's Guide to Everything, pg. 85: Perceiving A Caster at Work
If a sorcerer casts a spell with only verbal or somatic components using Subtle Spell, can an opponent use counterspell against it?
If a spell that’s altered by Subtle Spell has no material component, then it’s impossible for anyone to perceive the spell being cast. So, since you can’t see the casting, counterspell is of no use. Sage Advice Compendium, pg. 5
So it cannot be counterspelled.
Can be dispelled
Dispel magic is intended to break spells. Since phantasmal force cast using the lair action is indeed a spell, it can be dispelled by casting dispel magic on a creature affected by it. However, casting dispel magic on an affected creature only ends the spell on that creature. Phantasmal force affecting other creatures is not affected.
Dispel magic has a particular purpose: to break other spells. Sage Advice Compendium, pg. 14
Is subject to slow
The slow description says
If the creature attempts to cast a spell with a casting time of 1 action, roll a d20. On an 11 or higher, the spell doesn't take effect until the creature's next turn, and the creature must use its action on that turn to complete the spell. If it can't, the spell is wasted.
Aboleth attempts to cast a spell? Check. Phantasmal force casting time is 1 action? Check. Roll d20 and follow the description of slow. Note that Aboleth uses its action on its next turn, not next lair action "turn", to complete the spell.