When macro algae goes sexual, it is its way of reproducing. It releases algae spores so it can spread. In the wild this isnt a bad thing, but in the confines of a closed ecosystem it can cause problems.

When macro algae goes sexual, it releases everything that it has soaked up nutrient wise. If it did this as a slow/gradual release it wouldnt be a bad thing, but it releases them all at the same time. This can cause water parameter problems, resulting in a drop of ORP, rise in phosphates, etc.

Harvest a handful of the macro algae once a week, taking the harvest from the growing ends and it will help to keep it from going sexual. It will also stimulate it to grow faster.