A Crabby Encounter On The Way Home

If the weather is favorable, I walk home after work. I see many interesting critters as I move along passing the canal I've talked about, paddy fields, home gardens, and shrubs and  invasive  Pennisetum polystachyon growth on the road sides. It's on such a day that I saw these crabs in a rain-filled puddle near a paddy field. There were about ten, grabbing at the algae slime on the plants that were underwater or in contact with the water, with their pincers. I'm still trying to identify these as well as the one I saw at Thelagamu Oya River.

A Crab That I Saw At Thelagamu Oya

This is the only critter I had a chance to observe in our recent trip to Knuckles.  Could any of you help me to identify this one?