If you do water changes, you shouldnt really have to buffer anything to keep the pH in the 8.2 range. Do you do water changes on your tank? I would say do a 15% change and see what happens. Do this change with some RO water from the grocery store if you dont have a system (its drinking water, I believe it has a green top in most cases) and mix your salt so that you get a matching SG. Let the newly mixed salt water sit overnight with a powerhead or something to keep it moving, then do your change tomorrow.
The purple you are talking about could be one of two things. Coraline algea is an algea that is a good sign that your tank parameters are good. This algea grows in almost a circular pattern (ie starts as polka dots of purple) and will eventually cover your entire LR. The other thing it could be is cyanobacteria which is not a good thing, but it can be removed. Is the purple algea hard (can it be wiped off?). If it is hard it is coraline, if you can grap it and it comes up almost like a mat it is cyanobacteria.
The white stuff you are talking about could be a lot of different things. Do you dose with Calcium? Do you add salt to your display tank ever? If you dose calcium do you test for it? If not you could be overdosing and causing it to "snow" in the tank. Also adding salt mix to the tank without letting it disolve in water first could also cause this problem. Another thing it could potentially be is by adding a buffer for your pH. Do you add any buffers such as pH up, or pH 8.2? These can cause a snowing effect if they are not first dissolved in RO water.
Hope this helps. Answer a few of these questions above and we can probably narrow it down, also post your tank parameters (remember, we need actual numbers, not just "they're good").