Some things that come to mind:
Maybe the salinity was different from the starter culture you had and killed it off (I did this accidentally with an isochrysis starter that was done at 1.015 salinity, it died in 12 hours and smelled bad doing so).
Increase the airflow - you shouldn't need to stir it, really (nannochloropsis does settle out somewhat, but not quickly), but higher aeration will stir it for you and generally increase gas exchange which can increase maximum density (not relevant for where your culture is at, but helpful eventually).
Make sure to rinse the vessel thoroughly after sterilizing it so there isn't residual sterilizer in it.
What's the volume of your vessel? I'm culturing my phyto at around 4-5mL of fertilizer per gallon, so depending on the volume, it may be on the low side.
And finally, nannochloropsis doesn't really have that tint unless it's really diluted (and even then, I don't remember it being so yellow), so whatever you've got may not still be nanno. It's also one that is a little on the fragile side in terms of culture contamination, so other strains of things can take over relatively easily if cross contaminated (probably not this case, but worth knowing).
You can try upping your photoperiod to increase growth rate - 24 hours isn't actually a problem, but something like 18-20h will give great growth with a still present resting period.
Also, if you started with a small quantity of starter culture (like 20mL), I wouldn't expect it to grow to full density in only a week, especially if the fertilizer dosing is a bit light. While I still think I'd expect darker color than what you've got, it will take a starter a while to get developed enough to give consistently high density. In terms of density, you probably can leave about 1/6 of the culture remaining and still get consistent replacement growth in a week's time (I run 24h lighting, fairly high airflow, leave about 1/6 of a gallon for the next culture, add 5mL of fertilizer each reset, and can harvest the 5/6 of a gallon every 6 days.)