Yes, Nostoc in general is a colony in culture. What research you want to do with Nostoc, when the colony is a problem?

I want to research on nitrogen fixation and I have to equal amount of algae to compared the effect of my treatment. when it is colony how can I have equal amount of algae? If it isn't colony I can get OD for this case.

If I can understand, you want to use liquid medium for culture. As a solve of this problem I can propose to put equal number of cells if the volume of your experiment is not too big.

P.S. note that equal OD in some cases is not equal amount and also that fact that amaunt of algae always under dynamic change.

If the organism is forming colonies in the culture, easiest method to inoculate equal number of cells is to gently homogenize the cultures, get homogenous cultures and then add equal volume of culture.