I pick up the following paragraph from Agal culturing Techn. :

Ordinary agars consist of agarose and agaropectin that are contaminated with various impurities (Krieg and

Gerhardt 1981), and some agars contain water-soluble

lytic agents against cyanobacteria (Allen and Gorham

1981). For most currently cultured algae, general

purpose agar can be used without purification. For

sensitive algae, washing the agar is necessary to remove

the impurities (Carmichael and Gorham 1974,

Waterbury et al. 1986). "

Hi,

It has been show that the some bacteria form different colony morphology when exposed to different concentrations of agar.

Cyanobacteria are sensitive more than other common microbes  to standard agar, and when the agar is too dry or solid (high agar conc.) the tend to not grow well.