What is this?

[Jennyw]

It regrows quickly (within 24 hours), but wipes off easily

 

[ndrwater]

It's definitely a type of algae. Here in southern California we see that strain pop up starting g in the spring and into the summer months. Doesn't seem to be a whole lot that eats it with the exception of Sea Hares.
Wet skimming and more frequent water changes will help.

 

[Jennyw]

It's definitely a type of algae. Here in southern California we see that strain pop up starting g in the spring and into the summer months. Doesn't seem to be a whole lot that eats it with the exception of Sea Hares.
Wet skimming and more frequent water changes will help.

We have a skimmer and change 10-20% a week.

 

[ndrwater]

Are you using home made RODI? As a rule, I don't trust LFS water to be as good as it can be.
Skimming wet means that the skimmate should be more like tea and less like coffee.

 

[Jennyw]

I don't have a RO system, so yeah, I use it from the LFS. I've heard it's a good store, and salinity is always exactly right. It's a small tank, so installing a RODI system doesn't make a lot of sense...