looks like to me red sea lettuce depending on lighting it can be purple , red , or brown in color , It's a good macro algae to have in your tank , May need trimming over time
Common Name : Red sea lettuce
Origin : Caribbean
Nutrient intake :good
Maximum Height :18"
Growth : moderate
Light Moderate to low
Color : Red , Purple , brown
Palatable : yes
Many fish enjoy snacking on it