what are these bugs things and gray algae on live rock

Hi i recently left my tank to do its own thing only topping up water as i recently had my corals i add go down hill (hammer coral died so just grabbers from polyps left)
and since leaving the tank (no fish or corals other then 1 frag) the few cleanup crew i had in it seem to be breeding or something and was wondering is what i see eggs +babys or just pods

They move pretty fast and range upto about 2mm

here a pic of the baby looking things that recently appered above the eggs near the tank surface (the surface that the eggs on seems to have a thin algae layer to it which i guess is there food)
baby snails?

its hard to see in the pic but there the white dots when looked at close in tank they look like tiny feet of snails kinda like a bullet shape with 2 pointed feet

this is the "eggs" which appeared overnight about a week ago
http://ift.tt/2kVPz0I


Another question i have is i noticed a part of my live rock seems to have got grey algae like coraline algae is this good or bad or what is it i scratched it and it seems to be exactly the same as the red/purple stuff (i noticed recently all my rocks that had the coraline reds/purples exploded with spreading) here a pic

http://ift.tt/2kW6KPG