Kate explained "sleaze-bag". The term “alert” just emphasizes the word before it. An alert normally is a warning that something really bad and dangerous is coming and people should get ready for fight or flight.

It is a slang word that is defined as "a disreputable, disgusting, or despicable person," but in colloquial English it is almost always used in a way that it means "misogynist," or a man who does not treat women with any respect. SO to say "I mean sleazebag alert" (repeating it), you place extra emphasis on the fact that that character is REALLY disreputable. Further, in speech, you would want to emphasize the word "sleezebag."

Here is an article from Urban Dictionary that explains the usage of "I mean": https://www.urbandictionary.com/define.php?term=I%20mean Some more examples are given here: https://www.collinsdictionary.com/dictionary/english/i-mean The speaker is using it for clarification and emphasis: "He is a sleazebag. I mean, a REAL sleazebag!" The word "alert" is used as a noun when it is necessary to prepare for an imminent dangerous situation and people must be vigilant and cautious. For example, a "severe weather alert" might be issued by the authorities if a storm is expected with very high winds. So here the speaker is humorously suggesting that the person is such a sleazebag that it is necessary to issue an alert to warn people about it.

I wonder if it means that specifically the fact (among other things) that he has a ponytail is an evident sign of sleaziness, that the ponytail by itself makes her sleazebag alarm sound.

a disreputable, disgusting, or despicable person (also used as a general term of abuse) a slimy, untrustworthy person, usually male, especially in dealings with the opposite sex. Source:https://www.urbandictionary.com/define.php?term=sleazebag