Can I use Murphy's oil soap to remove pine sap from composite decking?
I have cleaned pine sap off deck chairs, deck table and cedar decking by first scraping with a plastic scraper. Then I wet a paper towel in turpentine and wipe off the remainder. Sometimes it is necessary to saturate the area to soften the sap so it can be removed by further scraping and wiping with the turpentine. 

The deck chairs I cleaned this way had plastic fabric seat material and were not noticeably affected by the turpentine. Of course after using the turpentine I followed up with a wash using warm soapy water to remove any of the chemical residue. 

I have every reason to believe that this cleaning technique would with the composite type deck materials. Since there are variations in material compositions it would be wise to check in a small inconspicuous area before committing the whole cleaning procedure.