Odd alge groth

[Zerandise]

I have got some odd rust brown/dard brown gunk growing on a small portion of my sand. Half of the substrate is 3 years old the other half is new. As well I just added in 60 lbs of rock i have cured in with the 40 i already have.

The groth is making what looks to be a slime *shell* over parts of the substrate.

I will post a pic of it as soon as i can. Has me a bit worries. My water tests perfect in every way as well (even in iron) so im not sure what it is. It had run fine for ever tho.