The addition of seaweed extracts was found to regulate the viability of cultures of the crustose coralline alga Lithophyllum yessoense. The viability was quantitated using a triphenyltetrazolium chloride assay, and the methanol-soluble extracts from 18 prevalent seaweed species were tested. Extracts from Codium fragile and Enteromorpha linza inhibited viability, and a Hizikia fusiform is extract slightly increased viability. The methanol extract of C. fragile, which had the strongest inhibitory activity, decreased viability to 72 or $52\%$ that of the control following addition of 0.2 or 2 mg/mL of extract to the culture, respectively. The main active compound in the C. fragile was lipid. This information is a preliminary result related to the exploration of seaweed restoration in the algal whitening area.