Empty the strawberry agar-agar into a large heatproof bowl. Pour 1 cup boiling water over the agar-agar. Whisk continuously until completely dissolved and smooth.

Stir in 1 cup cold water. Cover the bowl and refrigerate. Chill until the agar-agar is slightly thickened but not fully set, about 20 to 30 minutes.

While the agar-agar chills, hull and dice the strawberries. Drain the crushed pineapple very well in a fine sieve or paper towels to remove excess juice. Measure the marshmallows and whipped topping.

When the agar-agar has thickened, remove it from the fridge. Gently fold in the diced strawberries, mini marshmallows, whipped topping, and crushed pineapple. Add chopped nuts if using. Fold until evenly combined.

Pour the combined mixture into a large glass bowl or trifle dish. Smooth the top with a spatula. Refrigerate until fully set, roughly 2 hours.

Serve chilled. Spoon into bowls and garnish if desired.