What is this on my sand bed?

[Flame2hawk]

I’ve never had this before in all the decades I’ve been in hobby. What is it and how do I get rid of it…..

 

[MnFish1]

How long did it tank to develop? Have you tried siphoning it off? Like with a python or something like that?

 

[Flame2hawk]

Had it 3 months ago. Too heavy to siphon, Used a net and got as much of it as i could out. now its come back with a venguous. Its rather green and carpet like when the sand is bot on top. All my water parameters are good.