Spirogyra generally grows on plants in dense, medium to long curling threads.
It is usually caused by low CO2, excess organics (especially iron), too much light or poor water circulation.
To treat, start by manually removing as much as you can. If using pressurised CO2, reassess the amount you are injecting to see if it is sufficient. Clean your filter to reduce organic waste, and while the spirogyra is present, increase frequency of water changes – weekly if you are not already doing so, 2x per week otherwise.
If your photoperiod is longer than 7 hours or your lighting is particularly intense, reduce your schedule to 6 hours and observe over 2-3 weeks for any changes.
If you are using fertilisers containing Iron, consider adjusting your dosage rate or using a product containing less iron to reduce the levels in your aquarium water.
If circulation is poor, you can add a circulation pump, upgrade to a more powerful filter or trim overgrown plants to allow for better flow. Trimming plants has the added benefit of creating more bushy growth.
Treatment with products such as LCA Carbon Plus or Triple B is effective in weakening or inhibiting algae growth and may be suitable for combating spirogyra as part of a total treatment plan.