The answer to 'underlayment algae growth' is that algae growth on roofing underlayment is a common problem caused by moisture, shade, and organic debris. Symptoms include discoloration, slippery surfaces, and potential damage to the underlayment over time. To address this issue, you'll need to identify and eliminate the root causes, which may involve improving ventilation, removing debris, and potentially replacing the affected underlayment.
In many cases, minor algae growth can be treated with a DIY solution, such as cleaning the affected area with a diluted bleach or vinegar solution. However, if the problem is widespread or the underlayment is significantly damaged, it's best to call a professional roofing contractor to assess the situation and provide the appropriate solution, which may involve a full underlayment replacement.