Sulfurous Powder Problem

[9murphy9]

hi
im having a few problems with a light brown powder,in my tank i have block paving bricks in the middle of my tank as a tunnel/arch but today i noticed a sudden build up of this powder

http/s1237.photobucket.com/albums/ff473/9murphy9/?action=view&current=PA240298.jpg
http/s1237.photobucket.com/albums/ff473/9murphy9/?action=view&current=PA240299.jpg

is this the bricks dissolving as such ? or what ? pleas elet me know and if needs be ill get rid of em thanks for any help

Alex

 

[Aquascaper]

Google 'brown diatoms' and compare