Yes it can be.
Blue-Green algae means you have a healthy nutrient rich pond. All of this can change in a moment given wind and sunny conditions.
Keep your animals away if you see small green particles up to full blown blue/white foamy like substances on your pond. Dogs can gets sick and in many cases die if the pond is in bad shape.
Moderately harmful to Animals, ripe for pea-soup bloom. Easy to treat in 1-2 treatments.
Very harmful to Animals, ripe for full blue-green bloom. Easy to treat in 1-2 treatments.
Deadly harmful to Animals. Get Immediate help. Easy to treat in 1-2 treatments.