The brownish algae is probably either cyano
or diatoms. Are there bubbles trapped under the algae? If not, than you have diatoms which is completely normal. I am still battling them myself. They will go away on their own with time but you can add a clean-up crew to get rid of them as well. Turbos love that stuff.
Don't remember the rule for adding snails and crabs. I think it is around 1 for every 5 gallons or so. Hopefully someone else with a better memory will have an answer for you.
, don't put any inverts in until the tank is completely cycled. Ammonia and Nitrite will kill crabs and snails with ease.