I think of moonlight as like being under a space ship in Independence Day--if you see the big light shining down on you and you don't move quick enough, you get blasted.
That's...not what it says, though. It's if
you pass
into the beam, not if the beam passes
over you. So you could start in the circle, and then get knocked out of it by somebody else (accidental tactic snafu--quite common in any game with both the concepts of "forced movement" and "areas of effect"), and thus you
did in fact stand inside the beam, but nothing happened to you. Or the beam could pass completely over a particular monster--the "Independence Day laser" sweeps across the field, vaporizing anyone along the way. Except that that doesn't happen with Moonbeam--if the spell passes completely over a monster, then it remains unaffected, despite having completely failed to "move quick enough" to avoid getting blasted.
Well, no. You move on your turn. So if it's cast above you, then you start your turn under it and take the damage. This says you don't take the damage both when it's cast and when you then start your turn under it. It also makes it so that you can't be pushed back and forth across it on the same round, taking damage each time. So, you only take the damage 1/round, but you also can't jump in and out to avoid the damage on your turn.
Well, technically it says once per
turn, rather than once per
round, so the way he phrased it contradicts itself. Either it's only on that monster's turn, in which case it's once per round, or it's not only on that monster's turn, in which case the "once per turn" makes sense. Only if we assume he's saying once per
target's turn does it make sense...but that's a weird thing to mean when you say "once per turn."