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I like the idea of a Cloaker hiding on a fellow adventurer, nice touch with the half weight addition.
Engulf is unclear as to weather it needs to succeed on a touch attack before attempting to grapple an opponent. Grapples require touch attacks.
The ability to grapple, and attack other creatures without a penalty is very strong. While normally creatures with improved grab may grapple as if they were not at a -20 penalty, normal creatures cannot attack outside of the grapple at all. Consider allowing the attacks, but perhaps at a healthy penalty? Neither of its attacks are very powerful, and it can only do that when engulfing, which at first level means small creatures only. I don't see it as a major problem.
Engulfing leap mentions flight, which the cloaker does not yet have, it may be good to specify that it must land after this flying bout. It has to end the flying bout with an engulf attempt, it can end up anywhere as long as that anywhere is another creature's head.
It seems as if you are unsure when these abilities should be available without darkness. I am unsure whether they should be available without darkness. Removed that part.
The language in the moans alternates between mentioning enemies and anyone. I doubt a sonic-mind effecting screech could differentiate between ally and enemy, but if it can, you may wish to be consistent with it. Good point.
I suppose fear effects aren't terribly strong when everyone will be resisting or immune to them at this level, but still save or panicked is quite strong. It's only for two rounds and there will likely be at least one ally within 30 ft.
Stupor mentions level rather than HD, which implies that it comes into play at Cloaker L.9, which their isn't. You may wish to change that to HD. Right.
While some of the options are nice, it seems to me to be a very bare class, it could use some more fun abilities, maybe more fluffy ones that can play off the idea that it looks like a cloak, I mean, why not take advantage of the fact that a party member can wear it. I mean, think of all the awesome abilities a semi-magical living cloak that abhors nature could grant to your adventuring buddies!Consider it done