Please also excuse my criticism.
Return the clam as it likely will not survive. Some fish, coral and inverts require well established tanks and that is why they are not labeled 'easy to care for"
Clams are not easy and are not for beginners. It is a good idea to ask here before you buy.
If your LFS did not tell you that change LFS.

I think your goby should be fine provided that it eats prepared foods. My first fish was a yellow watchman goby and he survived all my mistakes and has moved from new tank to new tank without any problems. He won't even touch anything in the tank except the food that I feed him. He's actually kind of creepy because he knows that I'm the only one in the family that feeds him so he just sits there and stares at me for hours.