What is a prey of kelp?

Kelp is an autotroph, or a producer (they mean the same thing). This means that kelp produces its own food via photosynthesis. This is why it remains in areas where light penetrates the water easily. In other Words, kelp does not prey on anything.

On the other hand, if you are inquiring about what eats kelp, then you are looking for sea urchins, kelp's main predator.