The fish mask was a reward from the Squeal of Fortune minigame. It is a cosmetic item worn in the head slot that had an emote. Right-clicking the mask and selecting "Bubbles" performs a bubble emote.
The fish mask was initially introduced as an uncommon reward as part of a Squeal of Fortune promotion, the Squeal of Neptune, to accommodate a recent Fish Flingers update. It has been changed to a rare reward after said promotion's end, and will be made a tradeable rare item after the summer and therefore has a buying cap of two masks every four hours in the Grand Exchange.
Whilst Fishing with the fish mask, players may receive "compliments" from the fish. These appear as chat above the fishing spot, available for all players to see.
The compliments appear - around every 30 seconds - as follows: