Dirty Conch...

[shrimp]

I would just leave it or just take the shell out of your tank place in small container with your tank water grab a tooth brush and have at it.

[Dearth]

I get that on my snail shells occasionally after worrying about it the first few times I just wait it out now the other CUC clean it after a few days

[gregzz4]

If you have Turbo snails, just grab one and stick it on there
I do this when I have hair algae on my returns. When they're done, they find their way back

[Aquattro]

I wouldn't do anything. It's algae.