mushroomman has a good point: doyou have more than one of the same species of clown, and if so, is this one the largest one? if so, she may be spawning. the cleaning th algae being dangerous idea is that some algae absorbs high nitrates, and if you scrape it, these nitrates could leech back into the water causing another higher spike. as someone else said, a bad algae bloom generally means high nitrates, you may want to try another test kit, maybe yours is bad.
that said, the clown is one of the most resilient fish, I've cycled a new tank 3 times with the same false percula over the course of its 7 year life, they are tough. but if the nitrates spiked hard and heavy, this just be more than it can take. if you find high nitrates, a QT tank is a great way to treat, as someoen else already stated. good luck!