Is it ok to have a algae eater in my tank?

i have 4 ryukin goldfish and 2 common goldfish i was thinking of adding some oto catfish but if i want a algae eater, i want a good one. my friends dad gave me the tank and its kinda big, about 35-40 gall tank. does it matter if i have a fillter? and i have little white spots on the gl, is it dirty of is it just moss or algae