Again not sure you want to isolate or remove the starch. To remove it; use enzymes. To isolate you will need a fractionation protocol.
Do you want to isolate cellulose/starch from seaweeds that contain water-soluble sulfated polysaccharides or do you want to isolate cellulose/starch from a solution containing said polysaccharides?
case one, as already posted, follow cellulose extraction protocols. To obtain starch, you will need to design a "fractionating" extraction protocol.
case two, there should not be cellulose in such a solution, cellulose should stay behind in the un-extracted residue.
Again not sure you want to isolate or remove the starch. To remove it; use enzymes. To isolate; you will need a fractionation protocol.
Fully supporting the suggestions of David Myslabodski, I can add a note that if you mean a extraction of non esterified polysaccharides from sulfated ones, then you can isolate them after precipitating sulfate derivatives in their barium salt form and filtration.