Firstly, it is unsafe and not recommended to swim in a pool with algae, as algae contain toxins that can be harmful to bathers.
Algae can also cause staining on the surface of the pool, clog the filter and cause filtration issues.
If the Algae is not treated correctly you will have constant issues with keeping sufficient sanitisation levels in the pool. It is best to use the correct algaecide and companion chemicals to ensure that you eradicate the algae. If not, you will have reoccurring algae problems.