i can't say how the kelp will affect acne. i would suggest two things, first is to start out slow and work up from a small dosage. the second is to look at the ask moreless forum. there have been several threads about kelp and acne. i didn't follow the threads very closely, but at least a couple people posted their own experiences of using kelp and having Acne issues.
the nutrients in spirulina and kelp are different. spirulina is higher in beta carotene and chloropyl. kelp is higher in Iodine and mineral content.