Diatom blooms are very distinct. It is a brown "dust" that coats everything. It grows on the glas, sand, rocks, corals....and I'm sure if the fish stood still it would grow on them as well lol.
It is easily identified by touching it. It has a very slighty gritty feel to it and it rubs off easily.
I've never researched this, but I always thought it would be a bad idea to try to rub the stuff off of the "glass" if you have an acrylic tank. Diatoms, although microscopic, have very jagged edges and could possibly cause hazing on your acrylic if you rub them off.