I'm 4.5 month's in myself on the new tank (~15yrs exp) and so far I've had a couple outbreaks. It goes through a few phases and you may just be getting the last stages of the bacteria getting established. The thing that helped my tank the most was dosing live phytoplankton for about 5 days, then putting in a fuge once I got PO4 up. I had already put in pods a few weeks earlier, so adding the live phyto allowed them and the bacteria to flourish. Once they were outcompeting my diatoms, they were simply gone in days.
I'm currently growing a few SPS (among my Zs and Ps) and they are growing like weeds, I have had to start reducing my CUC as there's now so many pods and such in the tank there's nothing for them to eat. The pods are on the glass and walls 24/7 at this point, kinda nuts. In a good way.
What is needed is something to outcompete the diatoms, usually that means bringing your PO4 and NO3 up more so there is more nutrients in the tank for the other bacteria/pods to get going. I had run my NO3 at ~30 and PO4 and ~0.2 for a few weeks to get everything to out compete the diatoms. Once I had that started, I added a fuge to help reduce those back down. So far so good **knocks on wood**
What are your tank parameters? SG, dKH, Ca, PO4, NO3?
Edit: Also don't panic, diatoms are pretty harmless. If they get on your coral just blow them off with a turkey baster. If it's cyano or dino's, slight different story but similar treatment I believe. At least for Dino's for sure.
