Clam Boy is a monster that, due to having Transparency, can't be targeted until it attacks itself.
As such, while being unable to be targeted by enemy monsters, it also can't be targeted by the likes of Expulsion, Headshot and even Gaster Blasters, but also your own spells like Break and Another Chance. It will also not be able to be targeted by the abilities of monsters such as Vulkin and Nacarat Jester.
However, it is still able to be affected by non-targetable effects like Scarf Mouse, Woshua and Undyne. It also loses Transparency permenantly once it attacked, unless you return it to your hand/deck or use Asriel.
Also keep in mind that a monster with Taunt and Transparency will make Taunt not work until it loses Transparency (though due to being untargetable with Transparency, it can only have both if the enemy used Toriel or when you use Protection/Glad Dummy, followed by Asriel when it doesn't have Transparency).