Bubble Algae , Sailer's Eyeballs Scientific ID
Ventricaria ventricosa : grows to around 1" , clusters & singles
Valonia macrophysa : smaller clustered elongated bubble algae
Boergesenia : clustered but less dense in population with elongated bubble shape
Dictosphaeria ocellata : smaller grouped with slimey appearance lighter green
Dictosphaeria cavernosa : encrusting growth forms with pitted bubbles Good/Bad
Bubble shaped algaes that can easily spread throughout Rockwork which can be problematic to get rid of as some require very little nutriants . It can be removed by gently levering out of position but must be kept whole as it can release spores when burst which will grow more algae .
Natural predators include Emerald crab , rabbit fish and some tangs which can be a hit or a miss .More images here