1. The Silthilar is affected normally by the Horrific Appearance.
Reading from the d20SRD's rules on swarms:
Swarms are dense masses of Fine, Diminutive, or Tiny creatures that would not be particularly dangerous in small groups, but can be terrible foes when gathered in sufficient numbers. For game purposes a swarm is defined as a single creature with a space of 10 feet—gigantic hordes are actually composed of dozens of swarms in close proximity. A swarm has a single pool of Hit Dice and hit points, a single initiative modifier, a single speed, and a single Armor Class. It makes saving throws as a single creature.
Since the swarm is considered a single creature mechanically, it either entirely passes or entirely fails any saving throws. As a result, if the Silthilar fails its saving throw, all parts of the swarm take the Strength damage. Since the Silthilar in swarm form has 1 Strength, this would necessarily reduce the swarm to 0 Strength, and cause it to fall helpless.
Once the Silthilar is reduced to 0 Strength from the ghost, it is considered helpless. Being helpless also counts as being paralyzed at Dexterity 0. Lords of Madness gives us this:
Coalesced Form (Su): As
a standard action that provokes
attacks of opportunity,
a silthilar can switch from
its swarm form to its solid
... Paralyzed creatures cannot move, speak, or take any physical actions. The creature is rooted to the spot, frozen and helpless. Not even friends can move his limbs. He may take purely mental actions, such as casting a spell with no components. ...
Being paralyzed prevents actions from being taken, so the Silthilar could not coalesce to restore itself to above 0 Strength. However, since the swarm is composed of Fine creatures, and therefore immune to weapon damage and single target spells, the ghost would be unable to kill the Silthilar without the use of area of effect spells or Draining Touch to force the Silthilar down to 0 Constitution.