I believe cleaner shrimp will go for the trap too. Don't know about yours, but cleaner shrimps I kept are like pigs. They smell your food pellets on the LR or sand bed & approaches within the minute. However, cleaner shrimps are easier to catch if you have a net.
Option 1:
I've notice cleaner shrimps use their tails to shoot thru' the water when cornered & trying to make a quick escape. Position some food on top of your LR. Use net to scoop up shrimp when it tries to feed. Then house in beta box until crab is caught.
Option 2:
Position food at corner of tank on sand bed. Trap shrimp with net when it comes out to feed. Then house in beta box until crab is caught.
Note: If your fish net is small, You may net 2 nets to corner the shrimp.