That's what's caught my eye. If that goes live as is, it would make Mushrooms potentially very strong. Would be curious to see how much this gets adjusted before 5.2 goes live. If this ends up being the final version of the spell (doubtful) then that's potentially a mechanic for a druid to top up the entire raidWild Mushroom now has a Resto specific tooltip: The mushrooms grow larger as they accumulate healing power from 75% of overhealing done by your Rejuvenation, up to a maximum of 100% of your health in bonus healing.
It could of course mean that the bonus healing is split between the targets, but even if that is the case it's still potentially a v. nice change to the mechanic, would give a decent mechanic for healing burst raid damage (thinking Vizier, Garalon, to a lesser extent Spirit Kings, Feng here in particular)