Unless you can identify the species, never add crabs to the tank. Even some hermits can end up being regreted. Always drip acclimate inverts as long as possible. It will greatly increase the success rate. Most inverts are not a concern with bioload they actually help reduce the pollutants that degrade water quality.
As far as your algae problems, a good mix of snails will be your best solution along with proper RO
water and careful feedings. In newer tanks diotoms are quite normal. Once the silicates that fuel the growth is used up, the algae will crash on it's own. As long as you do not re-introduce the silicates, the diatoms should abate.