You should start your work with a basic search in Google: "Algenic acid HPLC". With just a few keytaps you'll get a buch of a suggestions how to proceed. Of course these may not be the best methods to proceed, but at least these give you the starting point. A step further would be a search in Webofknowledge.com database. Unfortunately that on is not a free tool, however lots of universities should have the access to this database. And if you've mapped the possible methods, then it's all about just testing and fitting the method for your own needs for samples and matrixes.
Such as:
H. Awad, 2012, "A Validated HPLC Assay Method for the Determination of Sodium Alginate in Pharmaceutical Formulations"
R.V. Rele, 2010, "Reversed Phase High Presssure Liquid Chromatograhphic Technique for Determination of Sodium Alginate from Oral Suspension"