Wow, I was in the same boat. I have a 110gallon/no sump. Couldn't figure out why my NO3
were slightly elevated. Didn't concern me until the algae started showing up. Ran phosban, got chaeto, do 10% water change weekly, use RO
water, changed my lights, cut back on light cycle, feed every 2-3 days...nothing would lower my levels.
I had a couple local reefers look at my set-up and they ALL agree - it's my skimmer. Are you running the regular Remora or the Remora Pro? The Remora is rated at 75g, but from what I've learned - even running the Remora Pro on anything larger than 75g is pushing it. I have the Remora Pro on my 110 gallon, and I had no problems for a long time then all of a sudden my PO4
is 0.1 and my nitrates will never sink below 20-30. You've got 150 gallons of water in your system - that skimmer is undersized, others may be able to back me up here. If you're using the Remora, even AquaC (who notoriously pad their numbers) only rate it for a 75g.
If you have a sump, why not upgrade your skimmer? I'm in the process of upgrading mine. If you can fit it - ASM G3 would be great for your system and you can get one as cheap as $240-ish. Not too shabby.