Hi the build up you have is probably algae.
If you want to get rid of it a algae pad does the job.
But since you want an cleaning fish there is some things you need to know.
1.The size of the aquarium is impotant as some fish may be large and some fish may be small.
2.Type of tank mates. Tank mates must have the same behavior or the fish may get stressed.
3.Aquarium water. The water style (Fresh, brackish) may stress or not stress fish, for example algae eating catfish some times get stressed in brackish water.
There you have the tips now on to the list of alage eating fish-
Cae is a great algae eater for a 20 gal semi-agressive planted tank.
Rainbow shark great for 30 gal semi-agressive planted tank with no other sharks.
Otocinclus is a small peaceful algae eating catfish that is good for 10 gal community lightly planted aqaurium.
Pleco is not the best algae eater. when sold they are about 2" when they mature they can get over a foot. These guys are destructive in planted aqauriums and produce 12-14" of waste a day. I recommend them in a 55gal agressive non planted aqaurium.
Clown pleco are great for 10 gal lightly planted aqauriums because they are small.
Garra ruffa are good for 10 gal but need a school of 5.
twig catfish are forgoten plecos. they are small, long and produce little waste.
Molies, gupies, platies are great algae eaters, mollies such as the black molly are great.
Thats all hope it helps.
Check out my fish assistance website yourfishblog - Home page
for help, apps, tools and articals on fish.