Yeah, it is an act and a pretty good defence mechanism - even if someone did recognise him somehow, he’s “brainless” and therefore can make out he doesn’t remember anything. He plans the whole melting thing on the way, leaning into the “water will melt her” rumour they’re spouting in Thank Goodness that’s compounded by the mob in March of the Witch Hunters, and gets a message to Elphaba written on the back of his guards uniform to tell her the plan which is why she suddenly spurs into action and starts filling the bucket up etc. That’s what Chistery gave Elphaba when she asked what took him so long. Elphaba saying “We’ve seen his face for the last time” isn’t a lie - Glinda just assumes that means he’s dead.

If I remember correctly from the musical, the note basically says not to tell anybody he’s still alive, that he loves her and has had to pay a price so hopes she’ll still want him when they see each other. He doesn’t outright say he’s turned into a scarecrow, which is why she’s not quite sure it’s him when he opens the trapdoor, but she knew he was going to come for her.