You need to find the cause of it first. Easy to get rid of but will come right back if the same conditions exist. Is not normally found in tanks which have good plant growth
in them. Though frequent in start up tanks, it usually leaves on it's own if the tank is made healthy. So...
What size tank/exactly what light and for how long/what if any ferts, CO2 etc.
The more specific you are about these, the easier it is to see anything that may be the cause of the tank not being healthy.