The 17-year-old, who has not been named by police, was walking to a friend's house in Conway Close when the pervert struck.
Police said the man, who was carrying a silver video camera, may have filmed her reaction.
The incident happened at 6.30pm near Toft Road's junction with Bexton Lane - close to the spot where a young woman was raped in the late 1980s.
Although his face was hidden by a scarf, the victim described him as in his late 20s, about 5ft 8 with short dark hair.
Yesterday (Tuesday) police refused to link the incident with previous attacks on the Heath.
In July a half-naked 6ft man exposed himself to a mother and her daughter near Ladies Mile - a month after two similar incidents on the Heath.
"At the moment we think this is a one off," said a police spokesman.
If you can help police call 01625 536222.
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