I really do not know . It is a factor of lights , and chemistry( Ca / Alkalinity) . This tank has been set up about 4 years. Lights are four VHO and 4 400 watt metal halides. ( So lights are older than 4 years) . VHO are becoming hard to buy now, so I may go to LED by force. I have a tank , same size, at office that already is on LED. I think that tank has purple algae too. I will try to find pictures, or take new ones of it tomorrow. I spent so much initially on these lights at home, that I hate to abandon just to chase new thing. Although electricity and bulb replacement on this set up is probably crazy. If you get conditions right , you get purple algae, if not , you get hair algae.
By the way, the first time we plugged those 4 400 watt halide bulbs in , it tripped all breakers for half of house. Now I have 2 dedicated circuits for aquarium only. One for 4 400 watt halides, and one for everything else- VHO s, wave maker,pumps , calcium reactor, chiller, skimmer, etc.
The office tank never had the overkill lights , and actually the invertebrates look better in some ways there. Lots of pulsing Xenia, leather coral, zoanthids, torch, stuff like that.