はな の した が のびている (のばしている）
hana no shita ga nobiteiru (nobashiteiru)
To ease your transition into non-flower expressions, let's go with one that uses the other はな: nose.
Some might argue that this expression could have been saved for the eventual unveiling of the project Brett and I are always talking about, 「The Daily Yoji: After Dark」, which would be just like this site, except... not safe for work. But since it's pretty tame (when compared to some of the expressions, vocabulary, and ... onomatopoeia that we'd post on a NSFW site) and I've had three chances to use it since I learned it last week, I figured maybe we could throw it out there and test the waters a little bit. :)
That bit under your nose is getting longer.
｢伸ばす」and ｢伸びる」 can be used interchangeably here.
This is a hard one to define and translate for two reasons.
1: It's such a common place phrase in Japanese that looking it up, even in conjunction with 由来 or 意味 yields so many search results that digging through them to find useful ones is ridiculous.
2. We don't have a roughly equivalent expression in English.
This is said to somebody or about somebody who is leering, or engaging in lecherous fantasizing. If you come across something that's エッチ, and your reaction could be construed as interest, that bit under your nose is getting longer. If you're having a conversation about sex and you get... distracted, that bit under your nose gets longer.
I suppose that in the case of staring, something like "Pick your jaw up off the floor," or "Your eyes are about to come out of your head," would be appropriate. And then, ｢鼻の下が伸びている」 might be a Japanese woman's version of "Hey! My eyes are up here." But most of the incidences of use that I've come across in my long, below-the-nose growing career are jocular. If it's being said, it's probably being said in good humor. ｢鼻の下が長いお爺さん」 sounds much more friendly and familiar than ｢変態じじ」 for sure.
This one is probably also familiar to fans of manga and anime, because there's a visual convention for it as well. In drawings, artists will usually do two things to characters who are having すけべな thoughts: They will elongate the portion of the upper lip, just below the nose, and they will give the character a sudden nose bleed. Yes, sexual thoughts cause nose bleeds.
And I've never seen this one in comics but I found it out the hard way by spending almost a year explaining to people that my hair grows really fast. Fast growing hair is another sure sign of a horndog.