Sniperhank
Giant Clam Addict
This is pretty simple TBH. I'd bet you are using tap water. Or, if you are using RO/DI, then you need to change your filters. I just got done learning this the hard way and my tank got overrun just like that. Toss in some red slime remover and increase flow in the tank. This will "band-aid" the issue, but it is obvious that the nutrients in your water are not good.