Welcome to AA!
I have to point out, as I don't think it's been covered, a 10 gal
tank is much too small for any pleco or molly, and a red claw crab is a semi-terrestrial brackish water crab. This means it needs marine salt in the water and land access, or it won't live for long. If it can't get to air, it's desperate to get to it so it can dry out, and will probably do everything it can to espace the tank. I would recommend taking these fish and the crab back to the store and restocking with fish that can thrive in your tank. The last thing you want is the molly reproducing. Mollies and plecos both have large bio-loads, meaning they create a lot of waste that your filtration will not be able to keep up with. More fish = more waste = more toxins in the water poisoning your fish.
Are you testing the water for toxins like ammonia, nitrite, and nitrate?