Basic factors affecting the appearance of algae discoloration on the surface of the system are recognized effects of the external environment (external temperature and humidity, short- and long-term radiation, precipitation, wind and air pollution). Internal factors are often neglected by international technical documents on the evaluation of the effectiveness of resistance to biocorrosion of the External Thermal Insulation Composite System (ETICS). Based on literature data and in situ research, the basic internal factors responsible for the occurrence or intensification of the biocorrosion process were systematized. Internal factors were divided into two groups: (1) plaster properties and (2) solutions for material layouts and building details. The results of research on these factors indicate that they directly or indirectly influence the humidity condition of plaster and biocorrosion development is a consequence of this state. The opposite issue, the influence of biocorrosion on plaster properties, is analyzed only in patrial way.