The "Enemies Missing"-ping is (un)officially labeled the "Flameping". It's just so unnecessary to use when you can write a short sentence, asking politely what he is doing, if you must question anyone's decisions at all.
Anyone ping that twice or more to me, I WILL stop any action, fall back (back to the altar if needed), and write "the enemies aren't missing - they are right there, in front of you", taking any time needed to get that grammatically correct, not giving a damn about any repercussions.
You want to (read: must) comment on such things, instead write something like "is there a game of cards going on over at Jhin's place?"
Phrasing it as a joke is usually enough to let them get the point, but without trying to irritate them. That said, it does raise the question why you weren't minding your own lane, since you pinged them at all.
Lastly, and as a Nami main myself, I can tell a flaming Nami isn't a true support player - more like an auto-filled one.