Are you talking about cyanobacteria ? And what kinds of multi fry do you have ?
A relatively safe method of getting rid of cyanobacteria, is to do a water change- lower your water level by about 40-50%, with your filter off, and spot dose hydrogen peroxide at close range (with a syringe or test kit pipette) onto the cyanobacteria patches. Leave it sit for 30-45 minutes - you should begin to see bubbles rising off the cyano - means it's working. Leave it overnite & repeat the treatment a second day if need be. That should get rid of the cyano permanently.
Careful not to dose the h2o2 directly at any fish while you're doing it..
Then vacuum well and clean up any detritus/decomposing plant matter etc., and provide some form of water circulation throughout your tank - a small pump, airstone, spray bar, whatever - that should do the trick.