Brown Muddy Algae?

[jollyroger]

hi all
and thanks in advance for any advice

i have a 'sponge' back display on the back of my fish tank
but have notice lately is has brown 'muddy' in the sponge, if i sqeeze the sponge its very muddy brown
cant clean it off as the background is all sponge and i will damage it, any advice what to do to get rid of it, it is also some brown algae on filter tank box and on glass.

tank set up 2 years
in hallway so no sun light in its path, light on for 8-10 hours
plants plastic and rockery
Trigon 190
Volume: approx. 190 Litres
Measurements: 99 x 60 x 70 cm

fish 10 guppies
4 neons
2 oto's
1 siamese eater