Slim algae is primarily a cyano bacteria.
It thrives on phosphate and nitrates.
There are a few ways to combat it.
Lower lights levels or the duration will have a marginal effect cyano is notorious for growing in low light.
Lower nitrates and phosphate thru water changes. Adding additional plants to take up excess nutrients.
Feeding less or making sure that all food gets eaten are a few simple ways to combat it.
It can also be treated with the use or bacteria designed for septic systems.