The phantom fungus is a medium 2 HD plant. This in itself is interesting, as it gives one the many immunities inherent in the plant type (mind-affecting, crits, and almost all poisons, to name a few), while reducing the number of the horrible racial hit dice plants get (which are about equal to a warrior's).
Phantom fungi also have Greater Invisibility at-will, combined with a racial move silently bonus. They have only one natural attack, but getting more isn't too hard, and there's more than enough levels to set up a sneak attack build. The racial bonus to strength helps.
Speaking of racial boni, phantom fungi also get +6 constitution. Their dexterity and wisdom are average, though, and their charisma and intelligence take considerable hits (net stat total: +0).
For now, I'm going to give them +1 LA. Yes, invisibility and immunities are interesting and innovative, but the fungi come with a number of drawbacks (handlessness, low speed, abysmal intelligence) that I feel balance them out to the degree that only a small LA is necessary.
Phase spiders are next.