I believe I have ostreopsis dinos. They are primarily on the sand bed but I've lost two acropora. I started running carbon and vacuumed the sand every night. Po3 was 10 and Po4 was .1 They kept coming back every day for about a week. I bought a UV Sterilizer and it had an immediate impact. It didn't get rid of the problem but slowed it down so I only have to siphon about every third day. Not sure where to go from here but I might try adjusting the flow rate on the UV sterilizer then give it some time to see where the tank goes.
https://www.youtube.com/watch?v=1o2alraNmsY