This paper deals with a particular alteration, reddish orange in colour, found on stoneworks exposed to weathering and connected with photo-autotophic microorganisms. The study was performed on archaeological marble remains of the Museo Nazionale Romano in Rome, placed inside and outside Michelangelo's Cloister. Analysis of the algae responsible for the alteration, and test of the algicidal action of two chemical products, Fluometuron and Bromacil. Discussion of the results.