Deliberately not a replacement

Er, well, "If this would" is only used in replacement effects... Did you want "When ~ becomes"?

(~ can be used in place of {this}, CARDNAME, etc, and will automatically expand to the card's name. I'm meaning to add a reference for the things like that that are expanded: others are "ETBs", "UEOT" and a couple more. Happy to take more suggestions.)

I tried a couple of guesses at what would expand, I'd possibly have hit ~ eventually :)

A reference would be good though.

Another problem is that sadly this doesn't work the way you'd hope. If you flip it up in response to someone targeting it, it won't get the counter-and-counter. To do that, it'd have to say... either "0: Counter target spell or ability that targets ~. If you do, put a +1/+1 counter on ~." or: "If a spell or ability targeting ~ would resolve, instead counter it and put a +1/+1 counter on ~." That last one isn't as ugly as I'd expected, actually.

I personally thing it should be . Spells or abilities targeting Mana Eater cost more to cast. Whenever Mana Eater becomes the target of a spell or ability put a +1/+1 on Mana Eater. 1/1.

So now its a Slippery Bogle thats hungry for mana.

Or: Counter all spells and abilities that target Mana Eater unless their controller pays an extra ; put a +1/+1 counter on Mana Eater. Though as Alex said, this wouldn't work if morphed in response to a spell or ability targetting it unless it had an activation cost.

This is definately not a common card. Given its abiliy, it could be a hybrid costing or a multicolour costing quite easily.

As you say, 'that would target' means it's too late to unmorph it as a surprise counter. Which kinda removes the point of it.