By the looks of it, Staghorn algae.
The method I use to get rid of staghorn algae is to make sure my plants are growing healthy, co2 levels are good, ferts are correct, and then I attack areas that have it with a syringe and hydrogen peroxide directly into the algae using as little force as possible on the syringe so that the hydrogen peroxide can stay on the algae bit as well as possible.
I do this every day to the same area until all the staghorn algae is red and then I move to another area and repeat. You'll want to turn off any pumps and your filter while you do this and let the hydrogen peroxide sit for around 30-45 minutes before turning them back on.
The reason I split the tank up into areas is because you want to stick close to the recommended dose of the peroxide as possible to not cause other damage.
You'll want to use 1ml's of hydrogen peroxide per gallon that your tank holds.
Your setup getting staghorn algae aligns with the timeframe my tanks get hit with it, I believe through observation of my own tanks that it has something to do with unstable parameters which are at play this early in a tanks life cycle such as random ammonia spikes that can't be caught by the existing bacteria in the system yet.
The main and only way I've been able to push the staghorn algae on its way out the door is by getting my plants growing correctly and then killing it where ever it shows up again, nothing else has ever worked for me with this algae.
By getting the plants growing and eating up the nutrients they provide competition to the algae.
Cutting off the nutrients to the plants only allows the staghorn algae to happily grow without being challenged, meanwhile the plants are unhealthy.
Last edited by Quesenek; 08-17-2020 at 06:32 AM.