In what way are slime molds similar to kelp?

The answer is They are both colonial protists.

Molds and kelp belong to the Protista kingdom that contains eukaryotic organisms that do not belong to the Kingdoms of Fungi, Plantae or Animalia. Those organisms can be either multicelled (kelp, slime molds, red algae) or single-celled (Paramecium). Among multicelled protozoans, kelp and slime molds are similar because they aggregate into colonies.