Slime algae issue

[Texas Reefer]

Hi! This is my 14g Biocube that I've modded to include a sump. So, it no longer has a back chamber, instead it has an overflow and return.
I've run tests, and they all look ok. pH is 7.79, salinity is 1.026, all is 6.0, pO4 is 0.03, and no nitrate. Why do I have this stringy slimy algae?

 

[Jason mack]

It’s an unbalance in your tank I suspect .. I would definitely raise your nitrates a little and also your alk ..
I would syphon as much out as possible and try and add more flow too the sand bed ...

 

[Texas Reefer]

It’s an unbalance in your tank I suspect .. I would definitely raise your nitrates a little and also your alk ..
I would syphon as much out as possible and try and add more flow too the sand bed ...

Ok, thanks! How do I raise nitrates? I felt like your was feeding my 3 fish enough, but should I feed more?