I imagine it depends on the critter! I suppose they'd need to ingest food or material of some sort every now and then to restructure it into more of their own substance to make up for losses... however any sort of metabolism works, there's a simple matter of mass balance to be maintained if one assumes they lose material over time. Need to get it back from somewhere, otherwise your goo critter just gets smaller and smaller.
Although that all pretty quickly gets theoretical anyway, if you start thinking about how such a creature metabolises energy from various sources and converts it into their own matter, how they move, how they maintain consciousness and intelligence, etc... hehe. Fun to ponder a bit though!