Brown Skin on Back and eyes

[amkid247]

I recently bought three fire bellied toads and have them set up in a ten gallon tank.
i noticed when i bought them that one had brown patches of skin on his back and eye lids. now he doesnt act sick in any way that im noticing, and he eats just fine. i was just wondering what this could be? i havent been able to find any info on the internet so far on this.

P.S. the waters brown from some eco earth falling in it by accident, there is a filter in the tank.
and i included some pictures of the set up!