welcome to OFL.
Are we talking about suckermouthed armoured catfish aka plecos or "just" an algae eater?
I suppose it's pleco's you are talking about.
Plecos can survive under very harsh conditions and will eat any scraps that the other fish drop.
They also are ditrinivores, wich means they actually eat s#it...
So starvation is not very likely, unless you buy a fragile underfed pleco.
Some plecos will clean the algae from your tank, but with the 7 i had ,only one does a decent job of algae cleaning. But these algae will never be enough for a healthy pleco and they will need an addition diet as well.
Plus they need driftwood in the tank, for they need the tannins from the woodraspings to digest their food properly.
Do you have some driftwood for your fish?
Best way to get a pleco to eat is to feed it at night when the O's are asleep.
Wait a few hours after lights out and drop in some sinking algae wafers and see if the pleco takes those.
Matt's advice is very good, nothing beats the Hikari pleco wafers and they got all my plecos to eat the wafers, while they simply refused any other brands i tried before that.
Nowadays i feed a quarter of a zuchinni at a time and i attach that to a stone ,that's placed near the plecos cave.
The oscars will not be interrested in the zuchinni, and the pleco will eat in peace.
You can also try potatoe, cucumber, banana, couliflower, but in general zuchinni is your best bet.
Here's one of mine eating the zuchinni:
Until she'll be so full, that she looks like this, but keeps eating.
And i'd like to see the answers to agrajag's questions, that will help us help you the best way we can!