My experiences with the same light in my fuge. A few suggestions, but they are that, suggestions based on my personal research and experience.
I have an established system with Chaeto, Ulva and Gracelleria in my fuge with the same Kessil H380 and it grows like mad. I have a high bioload and feed heavily so it gets lots of nutrient to grow. My fuge is really copepod heavy and I feed Phytoplankton into the sump at least one a week.
I run the light from 8PM to 10AM, perhaps reduce your photo period while nutrient is not abundant.
Your tank is young, like others say I would feed the tank to get the fish pooping, that's the best good fertilizer. I don't like a lot of wasted food breakdown but I do love to see some fish poop! If you had to you could add some Nitrate and, if needed phosphate. I keep Brightwells nitrate and phosphate and dose if numbers go way down, but I have reached a balance and have not had to do that once I got my feeding right. I would only do that after extensive research if you couldn't get it going with increased feeding of the tank.
Does you skimmer remove a lot and do you use any other nutrient reduction method? Carbon dosing, NoPox etc? You could leave the skimmer off and see if that improves the growth, let the chaeto use the organic waste. My fuge is before my skimmer so it gets first pick of the yummy fish poop.
Also, someone else said the Kessil needed to be 2' away, Kessil recommends 1 - 2 feet. Thats a big difference. Mine is 12" at most. My system has 60g water volume. Its a Red Sea Reefer 250 with a custom sump. I run a full Triton method system as well.I have 11 fish of varying size and eating habits. Photo-bomber is Meep the tuxedo kitten, that is no more than 2 months growth with this light. I upgraded from the H80 Tuna Flora as I got the new sump and it is much deeper and I wasn't getting good light penetration.