Are my cleaners just lazy?

Hi I am new to keeping marine fish, I have a 20 gallon tank that's been running for well over a month, at the moment I have 2 Halloween hermit crabs, a cleaner shrimp and 1 turbo snail and one nassariss snail?? (I'm getting 2 clownfish next week) I've had an outbreak of green algae and some brown on the live sand, I was told they would clear it up but they all seem to favour one rock and one is just full of green algae, should I get more snails? Or will they eventually come down?

Thanks for your help

Dan