The ghost shrimp in my goldfish tank have never eaten alge or diutrius. They instead manage to compete with the goldfish for fish food. It's not uncommon to find one that has caught one of the pellets and going to town on it. (One of the funny things is that I feed the pellets for the common, as he can easily eat them, but I have to include flakes for the rosy because he can not eat the pellets as they are much bigger than his mouth. But the little ghost shrimp can catch and hold on to one of these pellets and feed off of it.)
In a seperate 5 gallon tank, I have a male betta and 6 amano shrimp. I never see the shrimp because they spend all day hiding under the drift wood. If the betta ever spots one of them out from under the wood, he will chase it until it hides from the betta back in the wood or under a leave. But the betta doesn't seem to actively hunt the shrimp. Actually, the betta knows right where the shrimp are. They all hide in a knot hole in the drift wood. The hole is big enough for the betta to swim through, and every once in a while, he will swim through the hole temporaroly clearing out the shrimp. However, the 6 shrimp I started with in the tank are making a mess out of the floor of the tank. I am using white sand as a substrait, and there are brown little pellets all over the floor of the tank. I'm not sure if this is shrimp poop (which I've never seen any shrimp poop in the goldfish tank) or if the shrimp are slowly plucking the underside of my my drift wood. (The drift wood was actually something I got at the LPS
from the critter section that I boiled and sank).
One other thing that seems to indicate that my betta is not interested in eating the shrimp... I keep finding the left-overs from the shrimp sheding, even after a weekend when the betta has not been fed. In the goldfish tank, these things get eaten up.
At this time, the amano shrimp are making such a mess of the tank, I'm trying to transfer some of them to the goldfish tank... but dang they are hard to catch. Spent 30 minutes the other night just to catch the 1st 2.