The mermaids are first seen stealing stuff in Bikini Bottom by singing. Their reason for stealing the objects is so they can find something "cool" to wear. When they sing their song backwards, it returns the enormous pile of the stolen objects back to Bikini Bottom. They then decide to go to the mall with Pearl, because they want to find out what a mall is. Patrick has a crush on the Mermaids. Unlike Mindy, the mermaids look confused seeing Patrick when he wants to hang out with them.
"Mermaid Man Begins" has Mermaid Man's version of his backstory including the Lead Mermaid. She saves him from drowning and gives him a magic sea star that allows him to breathe underwater. The blue-haired mermaid also helps. They have more realistic appearances in this episode, but exactly the same colors.
Like Mindy, Patrick has a crush on them because he loves mermaids.