I've got this growing on dead weed stalks from from before the pond filled with water.

What is it? Is it some kind of moss or algea? Is it a good or bad thing?

Moss no, algae yes. I know of no species of moss that grows under water. I'll let Bill Cody give you an educated guess on what type of algae it is, the pictures are too blurry to identify it.

Another pic of it from ice fishing camera where it is growing on the rock pile.

Most likely filamentous alge. See this.

https://forums.pondboss.com/ubbthreads.php?ubb=showflat&Number=92633#Post92633

That looks about right.

Is that good to have?

It's only bad when it gets too thick.

Yes it is probably one of the several genera of filamentous algae (FA) that prefers cool water. FA in your pics loos to be in early growth stages. There are cool water and warm water forms. Cool water FA often subsides as water warms up and if nutrients are still abundant warm water FA replaces the cool water forms that are dying back. Example: Some common species of cool water Spirogyra start growth in fall, subsist during winter and thrive as water warms in early spring. Then in June -July Spirogyra dies back and is often is replaced by Cladophora that is known as a 'phosphorus hog'. This cycle in most noticeable in ponds that do not have some form of rooted weed beds nor an abundance of Chara. These ponds rarely have predominant FA problems.

Your pond and all ponds will always have some form or type of FA as a pond inhabitant. Ponds and lakes are never completely rid of some stage of FA growth. FA can exist as small patches of unnoticeable growths.. FA has numerous ways of colonizing ponds. Waterfowl manure contains FA remnants and 'spores'. Plants including FA thrive on utilizing dissolved nutrients. IMO experience FA grows best when the available nutrients are not being used by some other forms of plants be it attached periphyton to submerged rooted higher plants. FA seems to live best when no plants or very few other types of plants are not present to compete with the FA. Some lakes will develop some FA in early Spring that later disappears when the weed beds start their spring/summer growth cycles. Lakes rarely have FA problems because the weed beds in lakes consume most of the dissolved nutrients thus no excess nutrients are present to grow FA. .

As Bill notes, there are different varieties of Spirogyra. British:

and American:

First musician: "What should we call the band?"

Second musician: "How about we name it after pond scum?"

First musician: "Brilliant!"

Second musician: "I also think we should get amplifiers that go up to 11."

Since Spyrogyra has been previously trade marked and we want to avoid any lawsuit, a better name might be Mougeotia (mew-gee-oh-sha) or Cladophora (Cla-doff-era).. A third option could be Zygnema (Zig-knee-ma). We shouldn't forget Ulothrix (U-low-thrix). All are big players in creating various types of green pond scum. Poles are open for voting.