Advice? String algae?

Most RO/DI units will not remove silica. The only one I'm aware of is the Spectrapure MaxCap. Thats what I run and it has a Silicabuster canister. So if there are silicates in the new water then it will just feed the Dino even more.

Remove Dino X from the tank?

Here is a thread of the guy using peroxide to get rid of dino:

Dinoflagellates my experience......h2o2 reefing tool!!!!!

 

A multi prong attach is the best way to get rid of dinos, if you believe they can be. There are thousands of different kinds of dinos, some good and some bad. Dino's have been around since the beginning of time and have many, many adaptive abilitites...one is to encyst. There are almost as many methods to kill them supposedly. Dinos come in on fish, corals...even transferred from tank to tank. Methods: blackouts, h2o2, natural, tear down tank, Cruz's method, Metro method, bleach method and many more! What works for one, doesn't work for others. Over the years, I have tried all the methods.

Dino's love iron so do not do WC's as it will make it worse(iron is in reef salt), they also like very clean water, carbon dosing (vodka,vinegar,biopellets) and several other things.

Cyano and spirulina ...which do you have? There are different methods for getting rid of it. Identify which you have by:
Take a portion of that red mat you see in your tank and place it into a cup with about 2 cups of tank water.
Now add 1ml of 3% hydrogen peroxide
Over the course of several hours you may start to see a change in the water and the color of your sample.
If in fact it is the common cyanobacteria and not spirulina the water will start to turn a pink color and the sample with start to turn a green color.
Now if it is spirulina there will be no change as h2o2 has little effect on the individual cells.

One can have cyano and dino's, spirulina & dinos, GHA & dinos.......To name a few scenerios. Identify what what you have and then act quickly and agressively, especially with dinos!

 

I used regular 3%. Xenia do not like peroxide, so be cautious if you have any in your tank. My kenya tree retracted a bit for a day or two as well.

 

Dose it in the sump. If you don't have a sump, dose it in the highest flow area so you do not have a concentrated cloud of it in the tank. If you want, use it in a little SW from the tank and again, dose it in a high flow area.

It's good to be skeptical since you are new to the hobby. Find someone you trust that will give you all sides of the situation you are facing...a mentor if you will.