My two terrapins seem to be getting dirty shells, like a green algie looking thing on theres shells, maybe due to the filter and pump being rubbish.

Wonderd if there was any safe way to give their shells a clean, they're only about 3 inchs so i dont wanna do them any harm.

Why don't you just get a bigger/better filter? and what sort of basking area do you have set up?

Im gonna get a bigger better filter soon, its just what i got with the set-up, I have a big rock for them to bask on and a lil ramp thing to a raised area above the water for them.

You can scrub their shells very gently with a soft child's toothbrush to remove the algae, use nothing more than water though.

A better filter should clear it up in the long term, a short term solution is to add algicide to the water, don't be tempted to rely on that though as it's simply masking a fundamental problem with your setup.

Is there algae in the tank generally, is it near a window?

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Not really near a window no, and i clean the tank out reguarly, normally once every 5-7 days.

Do you have a heat and UVB source focused on the basking area? Turtles are cold blooded and rely on us to provide them with the means to get their bodies to the right temperature. What are your basking temps?