Officers say that a man in his 50s, who was performing a sex act on himself, approached a young man.
The incident happened on Saturday (12) at around 12.30pm.
A spokesman for Dyfed Powys police said: "Police would like to identify and speak to a man who was in the area at the time. He was around 5ft 11inches tall and had grey/brown stubble. He was wearing a blue fleece jacket with a grey and blue baseball cap."
Anyone with information that could help officers with their investigation is asked to report it to Dyfed-Powys Police, by email at: firstname.lastname@example.org, or by calling 101. If you are deaf, hard of hearing, or speech impaired text the non-emergency number on 07811 311 908. Quote reference: DPP/3422/12/10/2019/02/C.