Clam Girl is one of the 14 new cards added in Beta 7.6, alongside Clam Boy
. Previous Versions can be found here
When Clam Girl is placed on the board, she will reduce the cost of all spells in your hand by 2. This can make her really good in decks that rely on spells, such as Justice and Perseverance.
She also reduces the cost of spells generated by other monsters, making her pair well with Trader Temmie, Sans and even Omega Flowey.
Her effect from Beta 8.3 was bugged, as it didn't work on spells generated from Trader Temmie. Beta 8.3.1 (a few hours later) fixed that.