You'll want to prioritize the big three, especially with a tank addition coming soon. Alk, phos, and Nitrates. Even salifert or red Sea will do. Stay away from api.
Have you added a bunch of dry rock or sand recently that could be leaching silicates and feeding diatoms?
That looks like dinos with a D to me, which if that's the case, you'll really want a magnified photo of to get a specific ID and treatment course.
Do you have UV to add? That won't hurt anything.
