We have to make it 100% clear whether or not this thing will actually physically attach itself to its victim. Especially the first version is somewhat ambiguous on the topic: "magically attach itself", "latched on", "releases its victim"...
The extra verbage in the MCA1 version makes it reasonably clear the moss never actually makes physical contact: "the Dungeon Master should not at any time inform the character why the checks are necessary [...] or should the DM identify the source of the energy drain"
Should we be more specific about the nature of the link between the moss and its victim? Maybe they're a visible link on the Astral or Ethereal?
Either way, we should change some of the wording above. Maybe something like this:
"If the check succeeds, the moss will continue to try to establish a link until the intended victim leaves its area of effect, requiring subsequent Constitution checks each round until that happens. If and when the victim fails a check, the moss is able to establish an ethereal feeding link, and it immediately begins feeding at a rate of 1 hp per round until the victim dies, until he leaves the area, or until the moss has consumed four times its own hit-point total in life energy (16 hit points), at which time it is sated and severs the link with its victim."
This version does make the feeding link visible through See Invisibility and True Seeing - a very plausible choice of spells if the party suddenly starts losing hp to some invisible foe...