The diatoms no but those usually go away on their own after the tank gets a little older also there are a tun of fish that like to eat it! The green algae it just depends on the type if its green spot that's a little harder to work with. I don't know anything about Phosguard I usually add more phosphates to get rid of GSA.
I would look at controlling the lighting first. Less light, less algae.
The lights are only on for 5 hrs a day