What does it mean for algae to go sexual?
 

What happens? Is it bad for the tank?

It means the algae enters sexual reproduction. The algae tissue falls apart, releasing gametes in the process.

Its bad for the tank since it releases a large amount of nutrients into the water. It can crash a tank

Thats why calupera should be last on your list of macro to use. When cheato goes sexual, its a little different. Cheato will break off little pieces as its form of going sexual.